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Overview and Recommendations
Background
- •Community-acquired pneumonia (CAP) is defined as an acute infection of the lung parenchyma acquired outside a hospital or long-term care facility, confirmed by a new pulmonary infiltrate on imaging and compatible clinical features. It is the leading infectious cause of hospitalization among U.S. adults, with an annual incidence of 24.8 per 10,000, rising to 164.3 per 10,000 in those aged ≥80 years.
- •The etiologic spectrum is dominated by Streptococcus pneumoniae (the most common bacterial cause), followed by Haemophilus influenzae, Mycoplasma pneumoniae, Legionella species, respiratory viruses (influenza, SARS-CoV-2, rhinovirus), and, in immunocompromised hosts, Pneumocystis jirovecii and fungi. Atypical pathogens (Mycoplasma, Chlamydophila, Legionella) account for 20-30% of cases and require distinct antibiotic coverage.
- •Key risk factors include smoking (OR 2-4), COPD, diabetes, chronic heart disease, alcoholism, poor dental hygiene, and aspiration risk (dysphagia, neurologic disease). HIV remains a dominant driver in sub-Saharan Africa (seroprevalence ~78% in hospitalized CAP). Smoking cessation and pneumococcal/influenza vaccination are the most impactful modifiable preventive measures, with pneumococcal vaccine reducing CAP in COPD patients (NNT = 21).
- •CAP is classified by etiology (typical vs. atypical), severity (outpatient, ward, ICU), host immune status, and aspiration risk. Aspiration CAP accounts for 25% of cases in older adults and independently doubles mortality. The historic category 'healthcare-associated pneumonia' (HCAP) has been abandoned due to lack of outcome benefit from broad-spectrum therapy; current guidelines favor individual risk assessment for multidrug-resistant pathogens.
- •The pathophysiology involves microbial invasion overcoming alveolar macrophage defenses, triggering innate immune activation, alveolar-capillary barrier disruption (driven by microenvironmental acidification to pH ~6), and systemic inflammation. Transcriptomic profiling identifies two sepsis response signatures: SRS1 (immunosuppressed phenotype, 41% of patients) with 14-day mortality HR 2.4, and SRS2 (inflamed). This host-response heterogeneity explains why mortality has remained static despite antibiotics.
- •Long-term outcomes are poor: over 9.8 years of follow-up, CAP survivors have an adjusted HR for death of 1.65, with 30-day readmission rate of 15.7%. Cardiovascular complications (myocardial infarction, new atrial fibrillation, heart failure) affect 18-30% of hospitalized patients and drive in-hospital and post-discharge mortality. Pneumolysin and hydrogen peroxide from S. pneumoniae directly invade myocardium, causing necroptosis and scarring.
Evaluation
- •Suspect CAP in any patient with acute cough, fever, dyspnea, pleuritic chest pain, or unexplained fatigue, symptoms typically evolve over 1-7 days. In elderly patients, fever and cough are often absent; instead, look for confusion, falls, functional decline, or anorexia. Auscultate for crackles, bronchial breath sounds, egophony, and dullness to percussion (LR+ 2-6 for bronchial breath sounds). Tachypnea >30/min, hypotension, or hypoxemia (SpO₂ <90%) signal severe disease.
- •Confirm the diagnosis with a chest radiograph showing a new infiltrate (lobar, multilobar, or interstitial). Up to 33% of infiltrates are missed on plain film; if clinical suspicion remains high despite a negative radiograph, obtain a chest CT or lung ultrasound. CT modifies management in 59% of cases, initiating antibiotics in 16% and discontinuing in 9%.
- •Assess severity immediately to determine site of care. Use CURB-65 (Confusion, Urea >7 mmol/L, Respiratory rate ≥30, Blood pressure <90/60, age ≥65), a score ≥2 recommends admission, ≥3 suggests ICU. The Pneumonia Severity Index (PSI) has higher sensitivity (NPV 0.98 for mortality) and is better for identifying low-risk outpatients. For ICU triage, the IDSA/ATS 2007 minor criteria (≥3 of 9) have sensitivity 94% for need for mechanical ventilation or vasopressors.
- •Order bilateral blood cultures only in severe CAP (ICU admission or septic shock), positivity rate ~10%. Obtain sputum Gram stain and culture if a purulent specimen can be collected. For all hospitalized patients, perform pneumococcal urinary antigen (sensitivity 60-75%, specificity 99.7%) and Legionella urinary antigen (serogroup 1, sensitivity ~70%). Add a respiratory viral PCR panel (nasopharyngeal swab) to identify influenza, SARS-CoV-2, RSV, and other viruses.
- •Check laboratory markers: CRP and procalcitonin (PCT) are complementary. In early presenters (<3 days of symptoms), CRP may be low and PCT high ; in later presenters, CRP rises and PCT falls. A PCT <0.25 ng/mL can help defer antibiotics in low-risk outpatients. Also obtain renal function (creatinine, urea), liver enzymes, and CBC. For hypoxemic patients, an arterial blood gas defines PaO₂/FiO₂ ratio (<300 indicates respiratory failure) and PaCO₂.
- •Consider additional testing in special populations: in immunocompromised hosts (HIV, transplant, neutropenia), broaden workup to include BAL with metagenomic NGS for Pneumocystis, CMV, fungi, and mycobacteria. In patients with aspiration risk factors (dysphagia, bedridden, enteral feeding), assess with volume-viscosity swallow test. For patients from endemic areas, add serology for Coccidioides or Histoplasma.
- •Evaluate for complications early: if a patient fails to stabilize by 48-72 hours, repeat blood cultures, obtain a chest CT, and consider bronchoscopy with BAL for culture and targeted NGS. Look for parapneumonic effusion (5-12% of CAP), empyema, lung abscess, or noninfectious mimics like vasculitis, organizing pneumonia, or pulmonary embolism.
- •Consider alternative diagnoses in non-responders: pulmonary embolism (especially if D-dimer >3.13 mg/L in elderly), heart failure, malignancy causing post-obstructive pneumonia, drug-induced pneumonitis, or granulomatous disease (TB, fungal). In patients with prior antibiotic use, structural lung disease (COPD, bronchiectasis), or chronic steroid use, consider Pseudomonas aeruginosa or MRSA, use the PES score to risk-stratify for these drug-resistant pathogens (NPV 98% at score <5).
- •Document clinical stability criteria daily: temperature ≤37.2°C, heart rate ≤100/min, respiratory rate ≤24/min, systolic BP ≥90 mm Hg, SpO₂ ≥90% on room air. Once stable for 48-72 hours, plan IV-to-oral switch and discharge. Procalcitonin guidance can shorten antibiotic duration: use PCT <0.1 µg/L to strongly discourage antibiotics, <0.25 µg/L to discourage, this reduces exposure by 48% without worsening outcomes.
Management
- •Start empiric antibiotics immediately after diagnosis, within 4 hours of presentation. For outpatients (CURB-65 0-1, PSI I-II): prescribe oral 1 g three times daily OR 200 mg loading then 100 mg twice daily for 5 days (minimum 3 days if clinically stable). Alternatively, 500 mg orally on day 1 then 250 mg days 2-5 can be used if local macrolide resistance rates are low.
- •For non-ICU hospitalized patients (CURB-65 2-3): initiate a beta-lactam plus a macrolide. Preferred: 2 g IV daily PLUS 500 mg IV daily. Alternatives: 1 g IV q8h, 1.5 g IV q6h, or 750 mg IV daily as monotherapy. Beta-lactam monotherapy is acceptable in patients without SIRS or ≥2, based on CAP-START trial noninferiority.
- •For ICU patients (severe CAP, CURB-65 ≥4, IDSA/ATS major criteria): initiate 2 g IV daily plus 500 mg IV daily OR 750 mg IV daily. The network meta-analysis identifies ceftriaxone plus levofloxacin as having highest probability for mortality benefit. For patients with risk factors for Pseudomonas (COPD, bronchiectasis, prior antibiotics, corticosteroids), use 4.5 g IV q6h or 2 g IV q8h plus an antipseudomonal fluoroquinolone (ciprofloxacin or levofloxacin).
- •In suspected MRSA (cavitary infiltrates, prior MRSA, IV drug use, end-stage renal disease), add 15-20 mg/kg IV q12h (target trough 15-20 µg/mL) or 600 mg IV q12h. Do not routinely cover MRSA or Pseudomonas in low-risk patients; the PES score (≥5: high risk) helps discriminate.
- •For severe CAP requiring ICU care, add adjunctive hydrocortisone 200 mg IV daily (continuous infusion or divided q6h) for 4-7 days, then taper. The CAPE COD trial demonstrated reduction in 28-day mortality from 11.9% to 6.2% (NNT = 18). The benefit is concentrated in patients with CRP >204 mg/L (OR 0.43 for mortality). Do NOT use corticosteroids in nonsevere CAP, prednisolone 40 mg daily increased late failure (19.2% vs 6.4%).
- •Provide oxygen to target SpO₂ ≥92% (PaO₂ ≥60 mm Hg). For moderate hypoxemic respiratory failure (PaO₂/FiO₂ 210-285), use helmet CPAP, median time to PaO₂/FiO₂ >315 is 1.5 h vs 48 h with standard oxygen. If CPAP fails or PaO₂/FiO₂ <150, escalate to noninvasive ventilation (NIV) or high-flow nasal oxygen. NIV reduces intubation risk (OR 0.26) and ICU mortality (OR 0.28) in CAP with ARDS.
- •For invasive mechanical ventilation, use lung-protective strategy: tidal volume 6 mL/kg predicted body weight, plateau pressure ≤30 cm H₂O. In severe ARDS, consider prone positioning and early neuromuscular blockade. In extreme cases, ECMO may be considered in specialist centers.
- •Monitor for clinical stability daily. Once stable for 48-72 h, switch from IV to oral antibiotics. Use procalcitonin guidance to limit duration: for nonsevere CAP, treat for 5 days (minimum 3) if stable; for severe CAP, treat for 5-7 days. Longer courses reserved for complications (empyema, lung abscess, necrotizing pneumonia). Do NOT extend beyond 7 days routinely, inflection point at 7.4 days associates with increased length of stay.
- •If no response by 72 h, evaluate for complications: repeat chest CT, bronchoscopy with BAL for culture and metagenomic NGS. Consider pleural drainage for parapneumonic effusion (chest tube with fibrinolytics: urokinase or alteplase). For empyema, VATS is preferred if fibrinolytics fail. Drainage of complicated effusion reduces hospital stay by ~5 days.
- •For patients with aspiration pneumonia (gravity-dependent opacity, dysphagia risk): cover anaerobes with 3 g IV q6h, , or 500 mg IV q8h added to a beta-lactam. Avoid clindamycin due to high resistance rates.
- •What NOT to do: Avoid NSAIDs in early CAP, they increase odds of pleuropulmonary complications (empyema, cavitation) 8-fold. Do not use benzodiazepines routinely, they increase CAP incidence (OR 1.54) and mortality (HR 1.22). Do not use corticosteroids for parapneumonic effusions (no benefit, trend toward harm). Do not use routine antifungal coverage in immunocompetent patients.
- •When to refer: Consider infectious disease consultation for immunocompromised hosts, patients with drug-resistant pathogens, or refractory cases. Consider pulmonary consultation for bronchoscopy, chest tube drainage, or advanced ventilatory support. For patients with functional decline after hospitalization, refer to pulmonary rehabilitation.
- •Discharge planning: Ensure all patients receive pneumococcal vaccination (PCV13 or PCV20 + PPSV23 per schedule) and influenza vaccination (in season). Assess cardiovascular risk: check high-sensitivity troponin and consider optimizing guideline-directed medical therapy for heart disease. Screen for dysphagia in elderly. Educate on smoking cessation and dental hygiene.
- •Antibiotic drug doses in special populations: Renal impairment, adjust beta-lactams, fluoroquinolones, vancomycin based on eGFR. Obesity, use actual body weight for aminoglycosides, adjusted body weight for vancomycin. Elderly, no specific dose changes for most beta-lactams and macrolides, but monitor renal function. Pregnancy, beta-lactams and azithromycin are safe; avoid tetracyclines and fluoroquinolones.
Board Review — High Yield
- •CURB-65, Clinical score for severity: Confusion, Urea >7, RR ≥30, BP <90/60, age ≥65; ≥2 recommends admission, ≥3 suggests ICU.
- •Procalcitonin (PCT), Biomarker to guide antibiotic initiation and duration; <0.25 ng/mL discourages antibiotics in low-risk outpatients; <0.1 µg/L strongly discourages.
- •CAPE COD trial, Demonstrated hydrocortisone 200 mg IV daily reduces 28-day mortality in severe CAP from 11.9% to 6.2% (NNT=18); benefit concentrated with CRP >204 mg/L.
- •PES score, Predicts drug-resistant etiology (Pseudomonas, ESBL, MRSA); score ≥5 had NPV 98% for ruling out non-core pathogens.
- •Long-term mortality, CAP carries HR 1.65 for death over 9.8 years vs controls; 30-day readmission rate 15.7%; need vaccination and CV risk modification after discharge.
- •Aspiration CAP, Independent predictor of mortality (HR 5.69); screen for dysphagia; cover anaerobes with ampicillin-sulbactam or metronidazole-beta-lactam combination.
- •SRS1 transcriptomic signature, Immunosuppressed phenotype in 41% of CAP patients; associated with higher 14-day mortality (HR 2.4); identifies candidates for immunomodulation.
- •Urinary antigen testing, Pneumococcal (sensitivity 60-75%, specificity 99.7%) and Legionella (serogroup 1) are rapid, noninvasive tests that increase diagnostic yield in hospitalized CAP.
- •Helmet CPAP, In moderate hypoxemic respiratory failure (PaO2/FiO2 210-285), improves oxygenation faster than standard oxygen (1.5 h vs 48 h to reach PaO2/FiO2 >315).
- •Pneumococcal vaccination NNT, In COPD, 21 patients need to be vaccinated to prevent one episode of CAP (OR 0.62). Influenza vaccine reduces symptomatic influenza by 70%.
Deep Dive — Evidence Details
1. Definition, Classification and Nomenclature
- ▸CAP is a clinical syndrome classified by acquisition setting, care location, severity, host response, and suspected or confirmed aetiology. [1,35]
- ▸Hospitalized CAP, ICU CAP, and severe CAP are overlapping but non-equivalent categories; ICU-based and ATS-based severe CAP definitions identify different populations. [428]
- ▸Aspiration-associated CAP, viral or pathogen-specific pneumonia, mixed infection, necrotizing pneumonia, and pediatric PARDS-associated phenotypes should be named when clinically supported. [34,35,425,426,431,432,435]
- ▸Machine-learning, metagenomic, and biomarker-defined phenotypes are emerging classification frameworks and are not yet universal diagnostic nomenclature. [5,40,425,432]
Definition
Community-acquired pneumonia (CAP) is best understood as an acute pneumonia syndrome acquired outside the hospital setting or not otherwise classified as healthcare-associated, with classification based on the clinical context, severity, host response, and suspected or demonstrated aetiology. The 2026 American Thoracic Society (ATS) guideline addresses diagnosis and management of CAP in adults and reflects contemporary recognition that diagnostic methods, including lung ultrasound, and treatment strategies continue to evolve. [1]A1c The 2025 French multidisciplinary guideline similarly defines its scope as CAP of presumed bacterial origin requiring outpatient, hospital, or critical-care management. [35]A1c
CAP is a syndromic diagnosis rather than a purely microbiological label. Pathogen-based classification may be limited because a causative organism is not always identified, while clinical metagenomics and other emerging technologies may reveal biologically meaningful heterogeneity beyond conventional culture-based categories. [1]A1c[40]B3b Host-response and inflammatory phenotypes can also distinguish clinically important subgroups, particularly in severe disease and in children. [5]B2b[40]B3b[425][432]
Core classification
CAP should first be classified by site and context of acquisition: outpatient CAP, hospitalized non-ICU CAP, and severe or ICU-level CAP. The French guideline explicitly frames CAP management across these three care settings. [35]A1c Hospital admission alone is not synonymous with severe CAP. In contemporary US data, severe community-acquired bacterial pneumonia (sCABP) has been operationalized either by ICU admission or by ATS major criteria, and these definitions identify overlapping but non-identical populations. [428]
Severity classification is clinically consequential. In the US study, 24.5% of CAP bacterial admissions met at least one sCABP definition; 51.2% met the ICU-based definition, whereas 78.0% met the ATS-based definition. [428] These findings demonstrate that “severe CAP,” “ICU CAP,” and “CAP requiring hospitalization” should not be used interchangeably. [428] Severity may also be estimated with established bedside scores such as CURB-65, although inflammatory indices—including neutrophil-to-lymphocyte and platelet-to-lymphocyte ratios—have been investigated as adjunctive markers rather than replacements for validated clinical assessment. [429][436]
Aetiological and clinical subtypes
CAP may be described as bacterial, viral, atypical, fungal, mixed, or pathogen-unknown according to available microbiological and clinical evidence. The 2025 French recommendations specifically address presumed bacterial CAP, whereas contemporary evidence emphasizes that CAP can arise from diverse pathogens and that mixed infections are clinically relevant, particularly in pediatric cohorts. [35]A1c[40]B3b[432] Mycoplasma pneumoniae is a recognized CAP pathogen and may be associated with extrapulmonary disease, including Mycoplasma pneumoniae-induced rash and mucositis (MIRM); therefore, CAP nomenclature should not imply that disease is confined to the lungs. [222]B2a
Viral pneumonia may be included within the broader CAP syndrome, but pathogen-specific entities should be named when established. CAP and COVID-19 pneumonia can show similar circulating cytokine profiles while involving divergent biological pathways, supporting separate pathogen-specific nomenclature when SARS-CoV-2 is confirmed. [435]C Influenza-associated pneumonia is another distinct etiological subtype; critically ill patients with influenza may develop influenza-associated pulmonary aspergillosis (IAPA), even without classical risk factors. [426]
Aspiration-associated CAP should be identified separately when aspiration is clinically implicated. Aspiration community-acquired pneumonia (ACAP) is common among older adults and has been associated with worse outcomes than nonaspiration CAP, including differences in hospital stay, ICU admission, and mortality. [34]B2a However, aspiration-induced lung injury after acute drug or ethanol poisoning is a related but not automatically synonymous diagnosis; in that setting, impaired airway reflexes and aspiration may produce lung injury that requires separate consideration from infectious CAP. [429]
Complicated and phenotype-based nomenclature
CAP may be further qualified by complications such as necrotizing pneumonia, respiratory failure, acute respiratory distress syndrome, or cardiovascular complications. Adult necrotizing pneumonia is an uncommon complication of CAP with substantial morbidity, prolonged hospitalization, and uncertain consensus regarding radiological criteria and antibiotic duration. [431] In children, inflammatory biomarker patterns at admission have been studied for associations with severe CAP and pediatric acute respiratory distress syndrome (PARDS), indicating that pediatric severity phenotypes should not simply be extrapolated from adult classifications. [425]
Modern classification increasingly incorporates host-response biology. Machine-learning models have been developed to quantify immune dysregulation across pneumonia and sepsis populations, potentially separating clinical severity from the underlying biological response. [5]B2b In severe CAP, metagenomic profiles have been used to identify microbiome-based subtypes, while pediatric models have integrated microbiology, inflammatory markers, and clinical features to identify mixed-infection phenotypes. [40]B3b[432] These approaches remain phenotyping and research frameworks rather than universally accepted nomenclature. [40]B3b[432]
Finally, CAP classification should recognize outcomes extending beyond the acute episode. Long-term mortality after hospitalization has prompted development of the 1-year Long-term Pneumonia Mortality Index, and hospitalization for CAP has been associated with subsequent noncommunicable disease risk and cardiovascular events in observational studies. [12]B2b[430][434] These findings support documenting CAP as an index event with potential long-term consequences, while avoiding the unsupported assumption that every later event is directly caused by pneumonia. [12]B2b[430][434]
| Classification axis | Useful terminology | Evidence-based qualifier |
|---|---|---|
| Care setting | Outpatient, hospitalized non-ICU, severe/ICU CAP | Care setting is central to guideline scope, but hospitalization alone does not define severe CAP. [35]A1c[428] |
| Severity | Severe CAP, ICU CAP, sCABP | ICU admission and ATS major-criteria definitions are not identical. [428] |
| Aetiology | Bacterial, viral, atypical, fungal, mixed, pathogen-unknown | Mixed infection and pathogen-specific entities are increasingly recognized. [35]A1c[40]B3b[426][432][435]C |
| Aspiration | Aspiration-associated CAP (ACAP) | Should be distinguished from noninfectious aspiration-induced lung injury when appropriate. [34]B2a[429] |
| Complication | Necrotizing pneumonia, PARDS-associated CAP | These terms describe complications or phenotype-linked severity, not separate acquisition settings. [425][431] |
| Biological phenotype | Immune-dysregulated, metagenomic, inflammatory, mixed-infection phenotype | Primarily emerging or research classifications. [5]B2b[40]B3b[425][432] |
2. Pathophysiology and Mechanism
- ▸CAP can produce restrictive pulmonary dysfunction and ventilation–perfusion imbalance, contributing to impaired oxygenation. [70]
- ▸Disease severity reflects interacting pulmonary, inflammatory, metabolic, vascular, and pathogen-specific mechanisms rather than a single pathway. [435,437,438]
- ▸Pentraxin-3, SpO2/FiO2, and mid-regional pro-adrenomedullin were among the most informative individual outcome-related parameters studied. [437]
- ▸Gut dysbiosis and depletion of short-chain-fatty-acid-producing organisms are associated with CAP, but causality remains uncertain. [441,443]
- ▸Bacterial–viral co-infection and Streptococcus anginosus group infection may promote severe disease or pleural complications. [439,444]
- ▸Respiratory failure occurred in **15%–20%** of hospitalized children with Mycoplasma pneumoniae pneumonia in the cited cohort. [511]
- ▸Excessive intravenous fluid exposure during the first **24 hours** may be associated with later respiratory deterioration in children, although observational data cannot prove causation. [445]
2.1 Core pulmonary pathophysiology
Community-acquired pneumonia (CAP) produces inflammatory injury within the lung that can impair pulmonary mechanics and gas exchange. In a randomized clinical trial, CAP was characterized by restrictive pulmonary dysfunction and ventilation–perfusion imbalance, providing a physiological basis for hypoxaemia and reduced cardiopulmonary performance. [70]A1b These abnormalities may persist beyond the initial infectious episode, because post-acute mortality studies identified clinically distinct high-risk clusters after the acute hospitalization. [83]B2b
The severity of CAP reflects interaction among pathogen-related injury, host inflammatory responses, epithelial and endothelial barrier dysfunction, impaired oxygen transfer, and systemic organ effects. A prospective biomarker study emphasized that the complex pathophysiology of CAP complicates diagnosis, severity assessment, and outcome prediction. [437] In older adults, stress hyperglycaemia was associated with delirium, while neutrophil-to-lymphocyte ratio and procalcitonin provided only limited mediation of this association, indicating that systemic metabolic and inflammatory responses are related but not interchangeable mechanisms. [438]
2.2 Innate immune activation and inflammatory injury
CAP is associated with broad immune activation, but the molecular response is not uniform across respiratory infections. Comparative cytokine profiling found that CAP and COVID-19 could show similar plasma cytokine profiles while exhibiting divergent underlying biological pathways, supporting pathogen-dependent differences in immune regulation and tissue injury. [435]C Pentraxin-3, oxygenation expressed as the SpO2/FiO2 ratio, and mid-regional pro-adrenomedullin were among the most informative individual parameters evaluated for clinical failure, linking systemic inflammatory or vascular stress with respiratory impairment and adverse outcomes. [437]
Inflammatory signaling may also be regulated at the post-transcriptional level. In patients with CAP, dysregulated miR-4492 was investigated in relation to TRAF6, a signaling adaptor involved in inflammatory responses; serum expression, diagnostic performance, prognosis, and a miR-4492–TRAF6 interaction were assessed using patient samples and bronchial epithelial-cell experiments. [440] In pediatric Mycoplasma pneumoniae pneumonia, the lncRNA ACTA2-AS1/miR-2278 axis was studied in serum, bronchoalveolar lavage fluid, and infected epithelial cells because of its apparent relationship to inflammation and epithelial-barrier integrity. [512]
Oxidative stress represents another component of CAP biology. A prospective case–control study examined paraoxonase-1 concentration and arylesterase and paraoxonase activities together with C-reactive protein, procalcitonin, C-C motif chemokine ligand 2, lipid profiles, and clinical severity scores, reflecting the interaction between redox imbalance, inflammation, and disease severity. [442] Lipidomic research likewise indicates that lower respiratory infections, including CAP, are accompanied by reproducible alterations in lipid metabolism; these changes may reflect host–pathogen interactions and may provide diagnostic or prognostic signals, although platforms, specimen types, and study populations remain heterogeneous. [82]D5
2.3 Pathogen interaction, co-infection, and tissue complications
The causative microbial landscape can modify the inflammatory phenotype and anatomical complications of CAP. Streptococcus anginosus group organisms, oral facultative anaerobes, were identified as an underrecognized cause of pneumonia and were associated with empyema, including cases with pure or mixed cultures. [439]C Staphylococcus aureus combined with respiratory-virus infection has been investigated as a clinically severe form of CAP in which bacterial–viral interaction may produce synergistic pathogenic effects and worse prognosis than infection with either category alone. [444]
In children with Mycoplasma pneumoniae pneumonia, respiratory failure occurred in 15%–20% of hospitalized cases in the cohort underlying development of a clinical prediction nomogram. [511] This supports a mechanism in which intense infection-related inflammation and epithelial injury can progress to clinically significant respiratory failure, although the prediction model itself does not establish causality. [511]
2.4 Gut–lung and metabolic mechanisms
CAP is associated with disruption of the gut–lung axis. Prospective 16S rRNA sequencing found reduced gut microbial alpha-diversity and significant compositional differences between patients with CAP and healthy controls; these changes were accompanied by systemic inflammatory measurements. [441] Metagenomic and metabolomic profiling similarly demonstrated lower microbial diversity and depletion of short-chain-fatty-acid-producing taxa, including Faecalibacterium, Ruminococcus, and Eubacterium, as well as several representative species. [443] These findings support a potential relationship between intestinal microbial ecology, microbial metabolites, systemic inflammation, and pulmonary host defense, but observational designs cannot determine whether dysbiosis is a cause, consequence, or treatment-related correlate of CAP. [441][443]
Metabolomic changes have also been examined therapeutically. A controlled clinical study of 100 patients evaluated antibiotics alone versus antibiotics combined with Pneumonia Compound Formulation and used metabolomics to explore mechanisms of response. [71]A1b A multicentre, double-blind trial evaluated Maxing Huoqiao granules as adjunctive therapy in nonsevere CAP receiving moxifloxacin for 7 days, with follow-up to 14 days; the study was designed to assess clinical efficacy and potential mechanisms rather than to establish a universal pathogenic pathway. [85]A1b
2.5 Physiological deterioration and treatment-related modifiers
Pulmonary inflammation may make gas exchange particularly sensitive to fluid loading. In hospitalized children, cumulative intravenous fluid exposure during the first 24 hours was evaluated against respiratory deterioration occurring from 24 to 72 hours, based on the premise that excessive fluid could worsen gas exchange in inflamed lungs. [445] This is an observational association and should not be interpreted as proof that fluid exposure independently causes deterioration. [445]
Restrictive dysfunction and ventilation–perfusion abnormalities are potentially modifiable physiological consequences. In a randomized trial of 30 CAP patients, 940-nm near-infrared photobiomodulation delivered for 15 minutes daily over 7 consecutive days, in addition to antibiotics and physiotherapy, was compared with conventional therapy alone; pulmonary function and hematological parameters were the principal outcomes. [70]A1b The intervention findings may inform mechanisms of recovery, but they do not demonstrate that photobiomodulation prevents infection or replaces antimicrobial treatment. [70]A1b
2.6 Integrated interpretation
Overall, current evidence supports CAP as a heterogeneous syndrome in which pathogen-specific effects, innate and metabolic inflammation, oxidative stress, epithelial-barrier disruption, microbiome alteration, and impaired pulmonary gas exchange interact. [435]C[437][441][442][443][444] Biomarkers such as pentraxin-3, pro-adrenomedullin, inflammatory indices, redox-related variables, and regulatory non-coding RNAs are promising mechanistic or prognostic candidates, but most supporting studies are observational or exploratory and require external validation before routine mechanistic interpretation. [437][440][442][512]
| Domain | Evidence in CAP | References |
|---|---|---|
| Pulmonary physiology | Restrictive dysfunction and ventilation–perfusion imbalance | [70]A1b |
| Inflammation and prognosis | PTX3, SpO2/FiO2, and MR-proADM associated with outcome assessment | [437] |
| Systemic effects | Stress hyperglycaemia associated with delirium; NLR and PCT showed limited mediation | [438] |
| Redox and lipid metabolism | PON1-related variables and lipidomic alterations investigated in relation to inflammation and severity | [82]D5[442] |
| Microbiome | Reduced diversity and depletion of SCFA-producing taxa | [441][443] |
| Pathogen interaction | SAG-associated empyema and bacterial–viral co-infection | [439]C[444] |
| Barrier and gene regulation | miR-4492/TRAF6 and ACTA2-AS1/miR-2278 pathways | [440][512] |
3. Epidemiology, Etiology and Risk Factors
- ▸CAP epidemiology varies substantially by age, geography, season, pandemic period, comorbidity, and disease severity. [449,451,515,520]
- ▸M. pneumoniae is a major pediatric CAP pathogen, with reported epidemic cycles of 3–7 years and marked resurgence in some Chinese cohorts after the COVID-19 period. [451,516]
- ▸hMPV is globally circulating and associated with CAP; prevalence and severity vary by age, region, season, and study period. [515]
- ▸Diabetes, frailty, advanced age, immune-inflammatory dysregulation, and organ dysfunction are important markers of severe disease or poor outcome. [456,517,522]
- ▸P. aeruginosa and M. pneumoniae are important considerations in severe or necrotizing CAP, but pathogen distributions are population-specific. [518,519]
- ▸Observational study designs and regional sampling limit causal inference and broad generalization. [451,454,516,517,518,520,522]
Epidemiology
Community-acquired pneumonia (CAP) remains a major cause of hospitalization and mortality, with substantial heterogeneity in outcomes across age groups, comorbidity profiles, healthcare settings, and socioeconomic circumstances. [520] In a prospective multicentre cohort of hospitalized adults, approximately one in five patients required intensive care, and delayed ICU admission was associated with increased risk. [449] Among older adults, CAP is associated with a particularly high clinical and economic burden; frailty, septic shock, and mortality may not be fully captured by conventional severity scores. [522]
Age is an important epidemiologic modifier. Studies of hospitalized older adults have focused on patients aged ≥60 years, while ICU cohorts have specifically evaluated patients aged ≥65 years because of their elevated risk of adverse outcomes. [522][458] In children, Mycoplasma pneumoniae pneumonia (MPP) epidemiology has shown marked fluctuation around the COVID-19 pandemic. A Northeast China cohort identified 17,505 hospitalized MPP cases from 2018–2023, with the highest annual case count in 2023. [451] In Baoding, China, MPP cases increased from 844 in 2022 to 6,346 in 2024, while the positivity rate remained relatively stable at 67.68%–77.72%; children aged 5–18 years had the highest positivity. [516] These findings support substantial post-pandemic resurgence or re-emergence of M. pneumoniae activity, although the cited data are geographically localized. [451][516]
Human metapneumovirus (hMPV) is also a globally circulating respiratory pathogen associated with acute respiratory infection and CAP. A systematic review and meta-analysis of studies published from 2001 through June 2025 evaluated pooled prevalence, case fatality, age-related severity, and seasonality across geographic regions and COVID-19 periods. [515] The available evidence indicates that hMPV epidemiology varies by age, region, season, and pandemic period; precise prevalence estimates should therefore be interpreted in relation to the population and diagnostic methods studied. [515]
Etiology
The etiologic spectrum of CAP includes typical bacteria, atypical bacteria, respiratory viruses, and polymicrobial infections, with pathogen distribution varying according to age, geography, outbreak conditions, disease severity, and diagnostic strategy. [452][515][516] In pediatric CAP, targeted next-generation sequencing was investigated as a comprehensive pathogen-detection approach in 838 children admitted between February 2023 and May 2025, reflecting the continued difficulty of establishing a microbiologic diagnosis using conventional testing alone. [452]
M. pneumoniae is a prominent cause of pediatric CAP and may produce epidemic cycles of approximately 3–7 years. [451] In a cohort of 13,534 children and adolescents with CAP in Baoding, M. pneumoniae detection was frequent, particularly among patients aged 5–18 years. [516] Macrolide-resistance-associated mutations and molecular genotypes were specifically assessed in children, indicating that resistance epidemiology is an important component of the etiologic assessment. [516] A separate study of 421 hospitalized children with MPP in Liaocheng evaluated 23S rRNA macrolide-resistance mutations, MLVA genotypes, inflammatory markers, and predictors of severe MPP, reinforcing the association between pathogen genotype, antimicrobial resistance, host inflammation, and clinical severity. [454]
Severe CAP may involve less common or more aggressive bacterial pathogens. In a multicentre cohort of 130 patients with Pseudomonas aeruginosa severe CAP, metagenomic sequencing was used to quantify pathogen burden and assess associated lung-microbiome alterations and mortality. [518] The study’s design supports investigation of P. aeruginosa burden as a prognostic and etiologic marker, but does not establish that quantitative burden independently causes death. [518] In pediatric necrotizing pneumonia, pathogens were identified in 95.2% of cases, and M. pneumoniae was the most frequently reported organism, accounting for 52.50% in the cited cohort. [519]
Risk factors for severe disease and poor outcome
Host factors associated with adverse CAP outcomes include advanced age, frailty, diabetes, comorbidity burden, immune-inflammatory dysregulation, and baseline organ dysfunction. [456][517][522] In 2,471 CAP patients, type 2 diabetes mellitus was present in 22.6% and was associated with higher rates of septic shock (6.4% vs 3.5%), respiratory failure (22.7% vs 16.5%), and mortality (8.4% vs 4.3%) compared with patients without diabetes. [456] These observational findings support diabetes as a clinically important risk marker, while residual confounding cannot be excluded. [456]
Frailty is particularly relevant in older adults. A locally constructed electronic medical record-based frailty index using 36 parameters was evaluated for its association with septic shock and all-cause mortality in older hospitalized patients with CAP. [522] In critically ill patients with severe CAP, an immune-inflammatory dysregulation score based on six prespecified biomarkers was developed and evaluated against conventional prognostic measures, reflecting biologic heterogeneity beyond comorbidity and organ dysfunction. [517]
Sociodemographic factors may also influence outcomes. A multicentre Ontario cohort of adults hospitalized with CAP from 2015–2024 examined ICU admission or in-hospital mortality across age strata 18–39, 40–64, and ≥65 years, incorporating clinical and sociodemographic variables. [520] The study emphasizes that outcome disparities may differ by age and should not be inferred solely from older-adult data. [520]
Radiologic and physiologic markers may identify additional high-risk patients. Pulmonary artery diameter measured on CT pulmonary angiography was evaluated in 243 pneumonia cases and showed a consistent association with mortality in retrospective analyses. [521] However, this marker remains prognostic rather than an established etiologic risk factor. [521] In older ICU patients, SMART-COP, CURB-65, SOFA, and APACHE II were compared for predicting ICU mortality, intubation, vasopressor requirement, and hospital-acquired pneumonia, highlighting the importance of systematic severity assessment in this population. [458]
Epidemiologic interpretation
Most cited evidence is observational, retrospective, single-center, or geographically constrained, particularly the pediatric M. pneumoniae and severe CAP studies. [451][452][454][516][517][518][519][520][521][522] Accordingly, reported pathogen frequencies and risk associations should not be generalized across regions or seasons without considering local surveillance, vaccination and testing practices, antimicrobial resistance, pandemic-period effects, and case-mix differences. [451][515][516]
| Population or factor | Finding | Reference |
|---|---|---|
| Hospitalized CAP | Approximately one in five patients required ICU-level care in the cited prospective cohort. | [449] |
| Older adults | Frailty was evaluated using a 36-parameter index for association with septic shock and mortality. | [522] |
| Type 2 diabetes | Present in 22.6% of 2,471 patients; associated with higher septic shock, respiratory failure, and mortality rates. | [456] |
| Pediatric MPP, Baoding | Cases increased from 844 in 2022 to 6,346 in 2024; positivity remained 67.68%–77.72%. | [516] |
| Pediatric MPP, Northeast China | 17,505 hospitalized cases were analyzed; the highest annual count occurred in 2023. | [451] |
| Necrotizing pneumonia | Pathogens identified in 95.2%; M. pneumoniae accounted for 52.50%. | [519] |
| Severe P. aeruginosa CAP | 130 patients were assessed using metagenomic pathogen quantification and microbiome analysis. | [518] |
4. Clinical Presentation
- ▸CAP presentation is nonspecific; emergency-department diagnosis may be discordant with the eventual diagnosis, so assessment should combine clinical, laboratory, imaging, and vital-sign data [466].
- ▸In a non-ICU hospitalized cohort, clinically significant oxygenation impairment was defined by a **PaO₂/FiO₂ ratio <300** [450].
- ▸Severe Chlamydia psittaci pneumonia models identify severity at presentation rather than predict progression, and the available evidence is hypothesis-generating [210].
- ▸Children with viral, Mycoplasma pneumoniae, and mixed-pathogen CAP may have overlapping presentations; co-detection and extrapulmonary manifestations can modify the clinical phenotype [213] [222] [467] [469].
- ▸Pleural complications, including pyopneumothorax, should prompt consideration of unusual organisms and relevant exposure or dental history [465].
4.1 General presentation and diagnostic uncertainty
Community-acquired pneumonia (CAP) is clinically heterogeneous, and its manifestations may overlap with other respiratory infections and noninfectious conditions. In an emergency-department cohort of 1,385 adults initially diagnosed with CAP, investigators specifically evaluated diagnostic concordance, alternative discharge diagnoses, and the clinical, laboratory, imaging, and vital-sign features associated with confirmed CAP, underscoring that an initial clinical label may not always represent pneumonia [466]. Clinical assessment should therefore integrate symptoms, vital signs, oxygenation, examination, laboratory findings, and chest imaging rather than relying on a single feature [466].
4.2 Adults hospitalized outside the ICU
The Italian SIS-NET study prospectively characterized 176 adults hospitalized with CAP outside the ICU who had a PaO₂/FiO₂ ratio <300, indicating clinically important impairment of oxygenation in this cohort [450]. The study collected demographic, clinical, laboratory, radiological, microbiological, treatment, and outcome data and evaluated rapid syndromic respiratory panels in a subset of patients [450]. These findings support describing hospitalized non-ICU CAP according to the combined burden of respiratory impairment, radiological disease, systemic findings, and microbiological features rather than by symptoms alone [450].
Rapid molecular testing may alter the characterization of presentation because it can identify respiratory pathogens in patients whose routine evaluation is unrevealing; however, the SIS-NET study assessed its effect on hospital outcomes, not merely diagnostic yield [450]. A separate retrospective comparison of targeted next-generation sequencing (tNGS) with conventional tests analyzed 380 bronchoalveolar-lavage-fluid samples from patients with CAP or severe CAP, reflecting the use of lower-respiratory-tract sampling when disease severity or diagnostic uncertainty warrants more intensive investigation [211]B2b.
4.3 Features associated with severe disease
Severity assessment should distinguish patients who are already severely ill at presentation from those at risk of later deterioration. In patients with confirmed Chlamydia psittaci pneumonia, a small double-center retrospective study emphasized that its proposed creatine-kinase/direct-bilirubin model was designed to identify severe pneumonia at presentation, not to predict progression from non-severe disease [210]B2b. Because only 36 hospitalized patients were included, with 11 classified as severe and 25 as non-severe, the model is hypothesis-generating and should not replace established clinical assessment [210]B2b.
Psittacosis may produce systemic manifestations and multi-organ dysfunction in addition to pulmonary disease, which may be particularly relevant when the respiratory presentation is accompanied by disproportionate biochemical abnormalities or a compatible zoonotic exposure history [210]B2b. In a temporally associated series of four non-severe patients, bronchoalveolar-lavage-fluid tNGS was used to detect C. psittaci in a resource-limited county hospital, illustrating that an atypical or epidemiologically clustered presentation may require specialized testing [267]C4.
Legionella pneumonia is another CAP presentation that can require pathogen-directed therapy; a multicenter cohort studied adults with laboratory-confirmed community-acquired Legionella pneumonia and examined the interval from presentation to effective treatment and its relationship with mortality [461]. The study included macrolides, fluoroquinolones, and doxycycline as effective Legionella-directed therapies, but the treatment-timing findings should be interpreted as outcome evidence rather than as a symptom-based diagnostic rule [461].
4.4 Pediatric CAP and pathogen-specific phenotypes
Pediatric CAP may present with overlapping clinical syndromes that make etiologic differentiation difficult. A study of hospitalized children with human metapneumovirus-positive CAP compared 632 mild and 246 severe cases and evaluated demographic, clinical, and laboratory characteristics and independent risk factors for severe disease [213]B2b. The investigators noted that human metapneumovirus presentation closely resembles that of other common respiratory viruses, making clinical diagnosis alone challenging [213]B2b. Similarly, a biomarker-signature study addressed the difficulty of distinguishing respiratory syncytial virus from Mycoplasma pneumoniae because both can cause overlapping respiratory symptoms in children with CAP [469].
Among children with M. pneumoniae pneumonia, a randomized multicenter trial enrolled 160 children aged 4–14 years with mild disease and evaluated adjunctive Lianhua Qingke tablets during 7 days of conventional therapy [145]A1b. The trial’s population represents a mild pediatric M. pneumoniae phenotype rather than severe CAP, and its primary relevance to presentation is the recognition of M. pneumoniae as an important pediatric CAP cause in the context of increasing macrolide resistance [145]A1b.
M. pneumoniae may also cause extrapulmonary mucocutaneous disease. A systematic review and case series examined M. pneumoniae-associated oral mucositis and the clinical entity of M. pneumoniae-induced rash and mucositis (MIRM), assessing reported clinical features, diagnostic criteria, and outcomes from studies published between 2015 and 2025 [222]B2a. Prominent mucositis, particularly when accompanied by limited cutaneous disease, should therefore prompt consideration of an extrapulmonary M. pneumoniae complication rather than being treated as an uncomplicated respiratory presentation [222]B2a.
4.5 Coinfection and uncommon complications
Respiratory-pathogen co-detection is common enough in hospitalized pediatric M. pneumoniae pneumonia to warrant explicit assessment: in a cohort of 3,081 children, 1,173 had monoinfection and 1,908 had co-detection [467]. The study evaluated associations with severe M. pneumoniae pneumonia, complications, healthcare utilization, and costs, indicating that co-detection may be clinically relevant to the severity phenotype [467]. Unsupervised clustering research further classified pediatric CAP using microbiological findings from bronchoalveolar lavage, inflammatory response, and clinical characteristics, reflecting substantial heterogeneity beyond a single-pathogen label [432].
Empyema and pyopneumothorax are severe pediatric complications of CAP. A case report described a 10-year-old with bilateral pneumonia and pyopneumothorax caused by the anaerobe Prevotella oris after tooth extraction 1 week before symptom onset, illustrating the importance of dental or aspiration-related history when chest pain or pleural complications accompany pneumonia [465].
Frank’s sign, an acquired diagonal earlobe crease, was investigated as a marker associated with cardiovascular events after pneumonia in a prospective cohort of adults hospitalized with CAP and followed for 18 months [212]B2b. The composite cardiovascular outcome included new atrial fibrillation/flutter and myocardial infarction, suggesting that cardiovascular risk assessment may be relevant during and after hospitalization, although Frank’s sign is not a diagnostic feature of CAP [212]B2b.
| Clinical context | Presentation-related implication | Evidence |
|---|---|---|
| Hospitalized CAP outside ICU | Assess oxygenation, radiology, laboratory findings, microbiology, and outcomes together; PaO₂/FiO₂ <300 characterized the studied cohort | [450] |
| Suspected C. psittaci pneumonia | Consider systemic or multi-organ involvement and zoonotic exposure; proposed biomarkers identify severity at presentation | [210]B2b [267]C4 |
| Pediatric M. pneumoniae pneumonia | May occur as mild CAP in children aged 4–14 years; extrapulmonary mucositis may indicate MIRM | [145]A1b [222]B2a |
| Pediatric CAP with co-detection | Mixed respiratory pathogens may be associated with a distinct severity and complication phenotype | [432] [467] |
| Pneumonia with pleural air and pus | Consider empyema or pyopneumothorax and uncommon anaerobic pathogens, particularly with recent dental history | [465] |
5. Diagnosis and Workup (Pulmonary Function, Bronchoscopy and Imaging Anchored)
- ▸Reassess the initial CAP diagnosis when clinical, laboratory, imaging, or physiologic findings are discordant; alternative diagnoses are common in emergency-department presentations. [466]
- ▸Use oxygenation, respiratory effort, hemodynamics, and validated severity assessment as the acute physiologic core; pulmonary function testing is not the primary acute diagnostic test. [471] [523] [530]
- ▸Use CT selectively for nondiagnostic radiography, complications, unusual focal disease, or suspected pulmonary embolism; do not substitute exploratory biomarkers for validated pathways. [265] [528]
- ▸Consider quantitative pneumococcal PCR, targeted sequencing, or mNGS when conventional testing is inadequate and the result could change management; interpret molecular results in the context of colonization and contamination. [525] [267] [529]
- ▸Reserve bronchoscopy with BAL for severe, progressive, refractory, immunocompromised, or diagnostically unresolved disease, and use BAL to evaluate atypical infection, mixed infection, hemorrhage, and unusual pathogens. [432] [527] [532]
- ▸Incorporate epidemiologic clues, including bird exposure, zoonotic risk, and post-pandemic pathogen shifts, into test selection. [267] [367] [451] [533]
Diagnostic framing
Community-acquired pneumonia (CAP) remains a clinical-radiological syndrome rather than a single-test diagnosis. Diagnostic assessment should establish whether a lower-respiratory infection is present, estimate severity, identify complications, and seek an etiologic diagnosis when the result is likely to change management. Diagnostic discordance is common: in a retrospective emergency-department study of 1,385 adults initially diagnosed with CAP, investigators specifically evaluated alternative discharge diagnoses and the clinical, laboratory, imaging, and vital-sign features associated with confirmed CAP, underscoring the need to reassess competing diagnoses rather than treating the initial label as definitive. [466]
The initial evaluation should document symptom trajectory, fever, respiratory rate, oxygenation, hemodynamic status, mental status, comorbidities, immune status, recent healthcare exposure, antimicrobial exposure, aspiration risk, and relevant animal or environmental exposures. Exposure history is particularly important when zoonotic pathogens are plausible: psittacosis may be overlooked because its manifestations are nonspecific, whereas bird exposure and compatible imaging can support targeted testing. [527]C [533]C Chlamydia psittaci should also be considered in patients with compatible epidemiology when routine testing is unrevealing; a small temporally associated series used BALF targeted next-generation sequencing (tNGS) to investigate suspected cases in a resource-limited setting. [267]C4
Pulmonary function and physiologic assessment
Pulmonary function testing is generally not the primary test for acute CAP diagnosis. In the acute setting, physiologic assessment should instead emphasize pulse oximetry, work of breathing, respiratory rate, gas exchange, and the need for ventilatory support. Pediatric severity prediction research has incorporated clinical variables, laboratory findings, demographic data, and chest radiographs to identify children at risk of severe disease, while another pediatric nomogram study evaluated clinical and epidemiologic predictors of severe CAP. [530] [471] These models should be viewed as risk-stratification tools requiring external validation, not substitutes for bedside assessment. [530] [471]
In children managed as outpatients, guidelines have traditionally emphasized physical examination rather than routine imaging; however, a prospective multicenter study of youths aged 3 months to 17 years evaluated the interrater reliability of examination findings, reflecting ongoing uncertainty about how consistently these findings are recognized. [523] Thus, examination should be interpreted in the context of oxygenation, respiratory effort, age, comorbidity, and clinical trajectory. [523]
Imaging
Chest radiography remains useful when the diagnosis is uncertain, disease is severe, complications are suspected, or an alternative diagnosis must be considered. Imaging should be interpreted alongside the clinical presentation because CAP can have atypical patterns. In an adult case, C. psittaci infection produced a solitary, approximately 4-cm round or oval right-lower-lung opacity with surrounding ground-glass change, demonstrating that spherical pneumonia can mimic a focal mass or other localized process. [265]C4
Computed tomography (CT) is particularly useful when radiography is nondiagnostic, complications are suspected, or the differential includes pulmonary embolism, abscess, empyema, or an unusual focal lesion. Acute pulmonary embolism and hospitalization-requiring CAP may have overlapping clinical, laboratory, and radiological features; a single-center exploratory study compared 48 CT-pulmonary-angiography-confirmed pulmonary embolism cases with 34 CAP cases and evaluated soluble CD40 ligand as a possible discriminator. [528]C This biomarker remains exploratory and should not replace validated diagnostic pathways or CT pulmonary angiography when pulmonary embolism is clinically suspected. [528]C
Pleural imaging should be obtained when there is concern for parapneumonic effusion or empyema. Research evaluating serum S100A12, serum amyloid A, C-reactive protein, and the neutrophil-to-lymphocyte ratio examined their potential value for early identification of acute empyema in patients with pneumonia; these findings are preliminary because the study was retrospective and involved 60 patients with pneumonia-associated empyema. [526]
Microbiologic testing
Routine microbiologic testing should be selected according to severity, epidemiology, immunocompromise, prior antimicrobial exposure, and the likelihood that a result will alter therapy. Conventional pneumococcal diagnostics have limited sensitivity. A prospective observational study evaluated quantitative PCR targeting lytA and piaB in nasopharyngeal, oropharyngeal, saliva, and sputum specimens from adults with CAP and controls, applying sample-specific low-cycle-threshold cutoffs to reduce misclassification from background carriage. [525] Molecular detection may improve pneumococcal diagnosis, but interpretation requires attention to specimen type, quantitative thresholds, and colonization. [525]
Targeted or metagenomic sequencing may be useful when standard testing is negative, the patient is severely ill, or an unusual pathogen is suspected. BALF tNGS was used to detect C. psittaci in a four-patient case series and covered 226 respiratory pathogens. [267]C4 BALF mNGS was also used to characterize pathogens in CAP patients with type 2 diabetes mellitus, a population in which opportunistic and atypical infections may be clinically relevant. [529] In a retrospective series of 40 mNGS-confirmed C. psittaci pneumonia cases, investigators described clinical, laboratory, imaging, and bronchoscopic characteristics and examined factors associated with severe CAP. [527]C These studies support selective use rather than indiscriminate testing, because sequencing results require clinical correlation and may detect colonization, contamination, or clinically incidental organisms. [267]C4 [529] [527]C
Bronchoscopy and BAL
Bronchoscopy with BAL should be reserved for severe, progressive, refractory, immunocompromised, or diagnostically unresolved disease when lower-respiratory sampling is expected to change management. BALF can support culture, targeted PCR, tNGS, and mNGS, particularly for atypical, zoonotic, mixed, or opportunistic infections. [267]C4 [527]C [529] Pediatric research has also used BAL microbiology, inflammatory markers, and clinical features to identify a mixed-infection phenotype in 305 children with CAP, illustrating the potential value of lower-airway sampling in selected hospitalized patients. [432]
BAL is also useful when the presentation may represent a noninfectious pulmonary emergency. A case of isolated diffuse alveolar hemorrhage caused by atypical leptospirosis, without jaundice or acute kidney injury, highlighted BALF mNGS as a potentially valuable diagnostic adjunct when conventional acute-phase testing is limited. [532]C In children with pyopneumothorax, BALF or other advanced molecular testing may identify unusual anaerobic pathogens such as Prevotella oris, although evidence is limited to case-based literature. [465]
Pediatric and pathogen-specific considerations
Mycoplasma pneumoniae and Chlamydia pneumoniae may show changing epidemiology after public-health interventions. A retrospective study of 17,505 hospitalized children with M. pneumoniae pneumonia from 2018–2023 reported the highest annual case count in 2023, while a Beijing cohort of 331 laboratory-confirmed C. pneumoniae pneumonia cases examined changes across pre-intervention, intervention, and post-intervention periods. [451] [367]B2b Molecular or serologic confirmation should therefore be interpreted in relation to age, local circulation, timing, and clinical syndrome. A machine-learning study sought to distinguish respiratory syncytial virus from M. pneumoniae using a blood-based signature, but such models remain adjunctive pending broader validation. [469]
Diagnostic stewardship should accompany microbiologic testing. In hospitalized patients with cancer, INSPIRE trial analyses evaluated computerized prompts providing patient- and pathogen-specific MDRO-risk estimates to reduce unnecessary extended-spectrum empiric antibiotics, supporting risk-informed rather than reflexive broad-spectrum treatment while diagnostic data are pending. [133]B2b
| Clinical question | Preferred diagnostic direction | Evidence |
|---|---|---|
| Is CAP present or is there an alternative diagnosis? | Integrate examination, physiology, laboratory data, and chest imaging; reassess discordant cases. | [466] [523] |
| Is disease severe or progressing? | Monitor oxygenation, respiratory effort, hemodynamics, and validated clinical risk features; use pediatric prediction models only as adjuncts. | [471] [530] |
| Is there an atypical focal pattern or complication? | Consider CT; evaluate pleura when effusion or empyema is suspected. | [265]C4 [526] [528]C |
| Is pneumococcal infection likely despite negative conventional testing? | Consider quantitative PCR using lytA/piaB with specimen-specific interpretation. | [525] |
| Is an unusual, zoonotic, mixed, or opportunistic pathogen suspected? | Consider BALF PCR, tNGS, or mNGS after assessing whether results will change management. | [267]C4 [432] [465] [527]C [529] [532]C |
6. Severity, Staging and Risk Stratification
- ▸Use dynamic reassessment combining physiology, organ dysfunction, comorbidity, frailty, biomarkers, and trajectory; do not rely on a single score. [449][517][522]
- ▸Approximately **1 in 5** patients in the cited hospitalized CAP cohort required ICU admission; delayed escalation increased risk. [449]
- ▸A locally derived **36-parameter** frailty index may complement risk assessment in adults aged **≥60 years**, but requires external validation. [522]
- ▸In children aged **<5 years**, routinely available inflammatory biomarkers were evaluated for severe CAP and PARDS risk, without established universal cutoffs. [425]
- ▸Necrotizing pneumonia, persistent organ dysfunction, or failure of medical therapy warrants multidisciplinary escalation; in one pediatric cohort, **10.55%** required surgery. [519]
- ▸Short-course adult antibiotic therapy is defined as **≤5 days**, but duration should follow clinical stability and disease severity. [448]
Scope and practical staging
Community-acquired pneumonia (CAP) should be staged dynamically rather than by a single admission label. Clinically useful categories are: uncomplicated outpatient disease; hospitalized CAP without initial organ support; severe CAP requiring intensive-care assessment or organ support; and complicated or progressive disease, including acute respiratory distress, septic shock, necrotizing pneumonia, or treatment failure. The available updated evidence supports combining bedside assessment, comorbidity and frailty evaluation, inflammatory and organ-dysfunction markers, and repeated reassessment rather than relying on one prognostic score. [449][517][522]
In the PROGRESS cohort, approximately 1 in 5 patients with CAP required ICU admission, and delayed ICU admission was associated with increased risk; the study therefore evaluated a five-gene blood transcriptomic signature for identifying patients at risk of organ dysfunction and death beyond conventional scores and routine clinical metrics. [449] This signature remains an investigational adjunct rather than a replacement for clinical assessment because the cited study describes prospective observational validation work, not a universally established bedside threshold. [449]
Adult severity assessment
For adults, initial risk assessment should document respiratory status, hemodynamic stability, mental status, renal function, inflammatory burden, comorbidities, and the need for escalating oxygen or ventilatory support. Conventional tools such as CURB-65 and the Pneumonia Severity Index remain reference comparators, but recent studies identify additional domains that may improve risk discrimination in selected populations. [449][521][522]
Frailty is particularly relevant in older adults. A locally constructed electronic-medical-record frailty index using 36 parameters was evaluated in patients aged ≥60 years for associations with septic shock and all-cause mortality. [522] Because this index was locally derived and retrospectively evaluated, it should be interpreted as complementary to established severity assessment rather than as a portable replacement score. [522] Sociodemographic and clinical factors were also examined in a multicentre Ontario cohort of hospitalized adults, with outcomes defined as ICU admission or in-hospital mortality and analyses stratified into 18–39, 40–64, and ≥65 years age groups. [520]
Pulmonary vascular involvement may provide additional prognostic information: in a retrospective CT pulmonary-angiography study, pulmonary artery diameter was measured by three methods, which showed strong inter-method agreement (ICC 0.88), and the principal measurement demonstrated a consistent association with mortality. [521] This finding is hypothesis-generating and should not be used alone to determine ICU admission or treatment limitation. [521]
For critically ill patients with severe CAP, immune dysregulation may add prognostic information beyond comorbidity and organ dysfunction. A retrospective cohort of 223 patients admitted to a respiratory ICU developed an Immune-Inflammatory Dysregulation Score using six prespecified biomarkers, bootstrap-based stability selection, and multivariable logistic regression to predict 28-day all-cause mortality. [517] The score requires external validation before routine adoption. [517] In a separate multicentre retrospective study of 130 patients with Pseudomonas aeruginosa severe CAP, metagenomic sequencing was used to quantify pathogen burden and evaluate links between bacterial abundance, lung-microbiome disruption, and mortality. [518] Quantitative pathogen burden is therefore a potential research-stage refinement for selected severe cases, not a validated general-population staging criterion. [518]
Pediatric severity and progression
Pediatric staging should account for age, respiratory effort, oxygenation, altered perfusion or consciousness, feeding or hydration, and the evolution of complications. A prospective cohort of children aged <5 years hospitalized with CAP evaluated seven routinely available admission biomarkers—including neutrophil-to-lymphocyte ratio, platelet-to-lymphocyte ratio, lymphocyte-to-monocyte ratio, systemic immune-inflammation index, systemic inflammation response index, C-reactive protein, and procalcitonin—for inflammatory phenotypes associated with severe CAP and in-hospital pediatric acute respiratory distress syndrome (PARDS). [425] These markers may support triage in resource-constrained settings, but the cited evidence does not establish universal cutoffs. [425]
A prospective emergency-department reliability study enrolled youths aged 3 months to 17 years with suspected lower-respiratory-tract infection and fever; it evaluated interrater reliability of physical-examination findings used to diagnose pediatric CAP. [523] Consequently, severity decisions should recognize that examination findings may vary between clinicians and should be integrated with the overall clinical trajectory rather than interpreted in isolation. [523]
Mycoplasma pneumoniae pneumonia has variable severity in children. A retrospective cohort of 17,505 hospitalized children aged 28 days to 14 years examined outcomes and associated factors before, during, and after the COVID-19 pandemic, while another cohort of 421 children with M. pneumoniae pneumonia assessed host inflammatory markers, macrolide-resistance mutations, and MLVA profiles as potential predictors of severe disease. [451][454] These findings support pathogen- and host-informed reassessment when clinical deterioration occurs, but neither study establishes a universal pediatric severity score. [451][454]
Complicated pediatric disease requires separate escalation pathways. In a cohort of 398 children with necrotizing pneumonia, 42 (10.55%) required surgery after failure of medical therapy; preoperative complications were reported in 83.33% of surgical patients, and M. pneumoniae was the most frequently identified pathogen at 52.50%. [519] Necrosis, persistent sepsis, loculated pleural disease, or failure of appropriate medical therapy should therefore trigger multidisciplinary review. [519]
Interpretation of associated evidence
Risk stratification should remain linked to management response. Short-course antibiotic therapy means ≤5 days, compared with long-course therapy of >5 days, in the updated adult randomized-trial meta-analysis; duration decisions should be based on clinical stability and severity rather than calendar time alone. [448] Prospective non-randomized evidence in 186 older hospitalized adults evaluated adjunctive Chaihu Guizhi Tang plus Maxing Shigan Tang after clinical assessment, with day-7 clinical cure and 72-hour response as outcomes; this intervention is not a validated severity classifier. [513] Real-world omadacycline studies in adults and older non-ICU inpatients evaluated clinical failure, mortality, response, and adverse events, including analyses by age and CURB-65 strata, but their observational designs limit causal or generalizable staging conclusions. [514][424]
Diagnostic advances may refine attribution without replacing severity assessment. Quantitative PCR targeting lytA and piaB was evaluated for pneumococcal pneumonia using stringent sample-specific cutoffs to reduce classification of background carriage, while targeted next-generation sequencing was studied in 838 children with CAP for pathogen detection. [525][452] Human metapneumovirus epidemiology, including prevalence, case-fatality, age-related severity, and seasonality, was synthesized in a systematic review and meta-analysis of 101 studies. [515] Blood-culture practice in uncomplicated pediatric CAP was evaluated across hospitals, excluding ICU and complicated cases, because routine testing has low clinical utility in that population. [524]C These diagnostic findings can inform etiologic risk assessment, but ICU triage remains determined by physiology, organ dysfunction, and trajectory. [449][517]
| Domain | Practical implication | Supporting evidence |
|---|---|---|
| ICU risk and trajectory | Reassess frequently for organ dysfunction, escalating respiratory support, and delayed ICU transfer. | [449] |
| Frailty and age | Add frailty and age-stratified clinical context, particularly in older adults. | [520][522] |
| Immune-inflammatory phenotype | Consider research-stage biomarker scores only as adjuncts in severe CAP. | [425][517] |
| Pulmonary and microbiologic burden | CT pulmonary artery diameter and quantitative pathogen burden may be prognostic adjuncts, not standalone staging tools. | [518][521] |
| Pediatric complications | Evaluate for PARDS, necrosis, and treatment failure when deterioration persists. | [425][519] |
7. Acute Management and Exacerbation Rescue
- ▸Reassess diagnosis and severity early because emergency-department CAP diagnoses may be discordant with final diagnoses [466].
- ▸Use oxygenation, respiratory effort, organ dysfunction, comorbidity, frailty, and trajectory; PaO2/FiO2 <300 identifies impaired oxygenation in a hospitalized non-ICU cohort [450].
- ▸Consider targeted MDRO-risk assessment and MRSA-nares PCR-guided vancomycin de-escalation rather than routine broad-spectrum coverage [133,134,464].
- ▸In improving children with severe, uncomplicated CAP, evaluate intravenous-to-oral step-down and shorter therapy using the PediCAP evidence base [472].
- ▸Corticosteroids may be considered selectively in severe or hypoxaemic adult CAP; evidence is heterogeneous and M. pneumoniae-specific results should not be generalized automatically [473,474,457].
- ▸Ivermectin, Lianhua Qingke, omega-3 supplementation, and omadacycline remain indication-specific or investigational rather than universal CAP rescue therapies [300,145,475,477].
Immediate assessment and disposition
Acute management should begin with repeated assessment of oxygenation, respiratory effort, mental status, perfusion, and organ dysfunction, while confirming that the working diagnosis is pneumonia rather than an alternative emergency. Diagnostic discordance is clinically important: an emergency-department diagnosis of CAP may subsequently prove to be another condition, reflecting overlapping and nonspecific presenting features [466]. In children, physical-examination findings are central to outpatient diagnosis, but interrater reliability was specifically evaluated in the PedCAPS cohort rather than assumed to be uniform across clinicians [523].
Severity assessment should combine physiology with comorbidity and frailty. In hospitalized adults outside the ICU, a PaO2/FiO2 ratio below 300 was used to identify a population with impaired oxygenation [450]. Hypercapnia in patients with acute exacerbation of COPD plus severe pneumonia was associated with increased ICU-admission risk and altered inflammatory responses; PaCO2 was categorized as >45 mmHg, 35–45 mmHg, or <35 mmHg in the study [462]. Among older adults, premorbid walking ability added prognostic information beyond conventional severity assessment, although the evidence was retrospective [500]. CURB-65 combined with the neutrophil-to-lymphocyte ratio or platelet-to-lymphocyte ratio showed improved mortality-risk discrimination in observational emergency cohorts, but these markers should support—not replace—clinical reassessment [478]C[479]. Urinary suPAR and NGAL were investigated as severity biomarkers across ward, oxygen-requiring, and high-dependency/ICU CAP, but they remain risk-stratification tools rather than established rescue triggers [470].
Empiric antimicrobial treatment and early stewardship
Start empiric antibacterial treatment promptly when bacterial CAP is sufficiently likely, then reassess the regimen when microbiology, clinical response, and local resistance risk become available. Extended-spectrum empiric therapy should not be automatic in adults with cancer: the INSPIRE secondary analysis evaluated patient- and pathogen-specific MDRO-risk prompts because overall MDRO prevalence was described as low and broad-spectrum prescribing was common [133]B2b. The RESPIRE score was derived and prospectively validated to identify adults at risk for MDR-CAP in a medium-to-high MDR-endemic setting; such tools may help target antipseudomonal or anti-MRSA therapy, but external validation and local epidemiology remain essential [464].
For ICU patients receiving empiric vancomycin for suspected CAP, the STOP-Vanc randomized trial evaluated MRSA-nares PCR testing as a de-escalation strategy and used vancomycin-free hours alive as its primary outcome [134]A1b. A negative test should therefore be considered within a structured antimicrobial review rather than treated as a substitute for cultures, clinical trajectory, or local test performance [134]A1b. Blood cultures have low expected utility in uncomplicated pediatric CAP; a large hospital-level study excluded complicated infection, ICU admission, complex chronic disease, and prolonged hospitalization when examining this question [524]C.
In children aged 2 months to 6 years hospitalized with severe, noncomplicated CAP, the PediCAP factorial randomized trial directly tested intravenous-to-oral step-down using amoxicillin or amoxicillin-clavulanate and shorter total antibiotic courses than the WHO-recommended 5 days of intravenous therapy [472]. The trial population required point-of-care CRP >10 mg/L and body weight 3–30 kg [472]. These data support active consideration of oral step-down and duration minimization once the child is clinically improving and able to absorb oral medication, while recognizing that the supplied abstract does not provide the trial’s complete noninferiority results or regimen-specific conclusions [472].
Respiratory support and exacerbation rescue
Hypoxaemic adults with M. pneumoniae CAP were randomized to adjunctive oral betamethasone or standard care; eligibility required SpO2 <93% and respiratory rate >20 breaths/min, and the tested regimen was 3 mg daily on days 1–2 followed by 2 mg daily on days 3–5 [473]. This disease-specific trial should not be extrapolated automatically to all CAP etiologies. A 20-trial meta-analysis involving 5,866 adults evaluated systemic corticosteroids for CAP mortality and morbidity, confirming that the overall evidence base is substantial but heterogeneous [474]. Choice between hydrocortisone and methylprednisolone remains uncertain because comparative data were retrospective and propensity-matched [457].
Do not use ivermectin routinely for hospitalized COVID-19 pneumonia on the basis of CAP management alone. REMAP-CAP evaluated ivermectin versus no ivermectin in critically and noncritically ill hospitalized patients, with respiratory- and cardiovascular-organ-support-free days through day 21 as the primary outcome [300]A1b. Adjunctive Lianhua Qingke tablets were studied for 7 days in a randomized placebo-controlled trial of 160 children aged 4–14 years with mild M. pneumoniae pneumonia; this is insufficient to define rescue therapy for severe disease [145]A1b. Intravenous omadacycline was retrospectively assessed in 8–16-year-old children with macrolide-unresponsive M. pneumoniae pneumonia after persistent fever following 72 hours of azithromycin; only 18 children received omadacycline versus 46 continuing azithromycin, limiting causal inference and safety conclusions [475]C.
Supportive and investigational adjuncts
Enteral omega-3 supplementation was tested in a small pilot study of 84 patients with severe pneumonia, using 3.50 g or 8.75 g daily for 7 days; reported reductions in ventilation duration and hospital costs require confirmation before routine use [477]C. Rapid molecular respiratory panels were examined in a prospective Italian cohort of non-ICU CAP with PaO2/FiO2 <300, but observational evidence does not establish that testing improves outcomes in every setting [450].
Reassess frequently for deterioration, treatment failure, resistant pathogens, respiratory failure, or an alternative diagnosis. The current evidence supports rapid stabilization, individualized antimicrobial selection, early narrowing or oral transition when safe, and selective adjunctive therapy rather than indiscriminate escalation [133]B2b[134]A1b[462][464][472].
| Decision point | Evidence-informed approach | Evidence |
|---|---|---|
| Suspected MDR-CAP | Use local epidemiology and validated risk assessment; avoid reflexive extended-spectrum therapy | [133]B2b[464] |
| Empiric vancomycin in ICU CAP | Obtain MRSA-nares PCR when available and incorporate it into structured de-escalation | [134]A1b |
| Pediatric severe CAP improving on IV therapy | Consider oral amoxicillin or amoxicillin-clavulanate step-down and shorter total duration when clinically appropriate | [472] |
| Hypoxaemic adult CAP | Evaluate selective corticosteroid use; M. pneumoniae trial eligibility included SpO2 <93% and respiratory rate >20/min | [473][474] |
| Suspected macrolide-unresponsive M. pneumoniae in children | Specialist-guided consideration of alternatives is required; omadacycline evidence is retrospective and limited | [475]C |
8. Long-term and Definitive Management
- ▸Use clinical response, microbiology, complications, and resistant-pathogen risk to determine oral step-down and total antibiotic duration; PediCAP specifically tested amoxicillin or co-amoxiclav step-down and shorter courses in children with severe uncomplicated CAP.[472]
- ▸De-escalate unnecessary broad-spectrum therapy using patient-specific risk assessment, MRSA nares PCR, probability-based test interpretation, and local resistance data.[133][134][306][464]
- ▸Corticosteroids, clarithromycin, ivermectin, intensified heparin, photobiomodulation, and traditional medicines are context-specific or investigational adjuncts rather than universal CAP therapy.[95][145][300][303][304][480]
- ▸Post-discharge care should include vaccination, cardiovascular and thromboembolic risk review, mortality-risk assessment, and attention to loneliness, vitamin D deficiency, functional status, and medication necessity.[12][302][482][483][484][206]
Treatment completion and antimicrobial stewardship
Definitive management begins with reassessment of the clinical response, microbiology, complications, and risk factors for resistant organisms. In children aged 2 months–6 years hospitalized with severe, uncomplicated CAP, the PediCAP factorial randomized trial evaluated intravenous-to-oral step-down with amoxicillin or co-amoxiclav and shorter total treatment durations than the 5-day intravenous regimen recommended by WHO; its findings are directly relevant to selecting the shortest safe course, although duration decisions should remain individualized when complications, persistent instability, or an alternative diagnosis is present.[472] Oral step-down should be considered once the patient is clinically improving and can absorb oral medication, with drug selection guided by microbiology and local resistance patterns.[472]
Extended-spectrum empiric therapy should not be continued solely because of a cancer diagnosis. In non-critically ill hospitalized adults with cancer, the INSPIRE trials evaluated computerized prompts that supplied patient- and pathogen-specific multidrug-resistant organism risk estimates to improve empiric antibiotic selection and reduce unnecessary extended-spectrum therapy.[133]B2b For patients receiving empiric vancomycin in the ICU, the STOP-Vanc randomized trial evaluated MRSA nasal PCR testing as a tool to guide de-escalation; this supports using a negative, appropriately timed test as part of a broader clinical and microbiologic decision rather than maintaining vancomycin automatically.[134]A1b A validated RESPIRE score has also been derived and prospectively validated to predict CAP caused by resistant pathogens in a setting with medium-to-high multidrug-resistance endemicity; external validation and local epidemiology are essential before applying it elsewhere.[464]
Stewardship interventions should interpret tests in clinical context. A randomized inpatient trial tested an electronic stewardship note translating low procalcitonin or a positive respiratory-virus result into an estimated post-test probability of bacterial pneumonia and corresponding antibiotic advice.[306]A1b In the outpatient setting, a randomized digital vignette study of French general practitioners compared clinical assessment alone with chest radiography, C-reactive protein, or both to reduce unnecessary antibiotic initiation for suspected CAP.[305]C4
Adjunctive therapies and treatments not routinely indicated
Adjunctive corticosteroids require restriction to carefully selected patients. A randomized trial in children with severe CAP evaluated intravenous dexamethasone for 5 days in addition to antimicrobials, assessing treatment failure at 72 hours, respiratory failure, ventilation, shock, mortality, length of stay, readmission, radiographic progression, and inflammatory markers.[480] Separately, a pragmatic Kenyan trial randomized adults without a clear steroid indication to standard care or 10 days of low-dose oral glucocorticoids, with 30-day mortality as the primary outcome; these pediatric and adult results should not be extrapolated across populations or severity groups.[95]A1b
Clarithromycin has been studied as an adjunct rather than simply as pathogen-directed antibacterial therapy. Molecular analysis of the ACCESS randomized trial reported earlier symptom resolution during the first 72 hours and prevention of progression to respiratory failure and secondary sepsis when clarithromycin was added to standard care in hospitalized CAP, with associated changes in inflammatory and immune pathways.[303]A1b This evidence does not establish routine clarithromycin use for every patient.
Ivermectin should not be added for hospitalized COVID-19 pneumonia solely on the basis of CAP terminology: the REMAP-CAP randomized platform trial compared ivermectin with no ivermectin in critically and noncritically ill patients, using respiratory- and cardiovascular-organ-support-free days through day 21 as the primary outcome.[300]A1b Intermediate-dose rather than standard low-dose heparin thromboprophylaxis was also evaluated in critically ill patients with COVID-19 for up to 14 days, with organ-support-free days as the primary outcome; anticoagulation intensity should therefore follow the patient’s thrombotic and bleeding risks and applicable COVID-19 protocols.[304]A1b
Photobiomodulation remains investigational. A small randomized trial of 30 CAP patients compared conventional therapy and physiotherapy with or without 940-nm infrared LED treatment delivered for 15 minutes daily for 7 days, assessing pulmonary function and hematologic parameters; this sample is insufficient to establish routine use.[70]A1b Lianhua Qingke tablets were evaluated for 7 days as adjunctive treatment in 160 children aged 4–14 years with mild Mycoplasma pneumoniae pneumonia; this single adjunctive trial does not replace antimicrobial therapy or justify general use outside the studied context.[145]A1b In elderly adults with CAP, a randomized study compared omadacycline with moxifloxacin; reported pathogens included viridans streptococci, Klebsiella pneumoniae, and Neisseria sicca, and clinical response was reported as greater with omadacycline, but the small sample limits definitive comparative conclusions.[337]A1b
Discharge planning, prevention, and long-term follow-up
Discharge planning should address functional recovery, medication reconciliation, vaccination, aspiration or swallowing risk, smoking and exposure reduction, nutrition, and timely review of persistent symptoms. Annual influenza vaccination remains important in older adults; a cluster randomized crossover study compared adjuvanted with high-dose inactivated influenza vaccines in adults aged ≥65 years using PCR-confirmed influenza as the endpoint.[302]A1b The study informs vaccine selection but does not support withholding either formulation when one is recommended and available.
CAP may mark increased future vascular risk. A systematic review found that adult CAP was independently associated with cardiovascular disease, including heart failure and acute coronary syndrome, as well as deep-vein thrombosis and pulmonary embolism.[482] Follow-up should therefore include assessment of new cardiopulmonary symptoms and optimization of cardiovascular risk factors. Long-term mortality can be substantial: the international L-PMI was derived and externally validated to predict 1-year mortality after CAP discharge across Spanish, German, US, and COVID-19 pneumonia cohorts.[12]B2b Older adults reporting loneliness had increased concern for adverse 180-day outcomes, including mortality and respiratory-infection readmission, supporting social and functional assessment during follow-up.[483]
Vitamin D status may contribute to risk stratification but is not a substitute for established treatment. In a prospective cohort of adults hospitalized with CAP, deficiency was defined as 25-hydroxyvitamin D <25 nmol/L, insufficiency as 25–<50 nmol/L, and sufficiency as ≥50 nmol/L; deficiency was associated with increased mortality risk.[484] Proton-pump inhibitors should be continued only when there is a clear indication, because an overview of systematic reviews found heterogeneous and inconsistent evidence regarding adverse drug reactions and safety outcomes.[206]D5
| Domain | Practical approach | Evidence |
|---|---|---|
| Antibiotic completion | Reassess stability, absorption, microbiology, and complications before oral step-down; avoid unnecessary prolonged or extended-spectrum therapy. | [472][133]B2b[306]A1b |
| Resistant-pathogen strategy | Use local epidemiology and validated risk tools; consider MRSA nares PCR to support vancomycin de-escalation. | [134]A1b[464] |
| Adjuncts | Restrict steroids and other adjunctive therapies to studied populations or specialist-directed indications. | [70]A1b[95]A1b[145]A1b[300]A1b[303]A1b[304]A1b[337]A1b[480] |
| Follow-up | Review recovery, cardiopulmonary symptoms, vascular risk, vaccination, social support, and 1-year mortality risk. | [12]B2b[302]A1b[482][483] |
| Medication safety | Reassess vitamin D deficiency and the ongoing indication for PPIs rather than prescribing routinely. | [206]D5[484] |
History and Evolution of Treatment
- ▸Modern CAP treatment is evolving toward shorter courses, oral step-down, and reassessment rather than automatic prolonged intravenous therapy [472].
- ▸Patient-specific MDRO risk, bacterial-pneumonia probability, respiratory-virus results, and MRSA nasal PCR can support empiric narrowing or discontinuation of antibiotics [133,134,306].
- ▸Rapid pathogen-directed diagnosis is increasingly important for Legionella, Chlamydia psittaci, and unusual anaerobic infections [461,465,485].
- ▸Necrotizing pneumonia and pyopneumothorax may require drainage or surgery when medical treatment fails or complications develop [465,519].
- ▸Adjunctive macrolide, photobiomodulation, omega-3, ivermectin, and anticoagulation strategies have been investigated, but their evidence and applicability differ substantially [70,300,303,304,477].
- ▸Modern treatment goals extend beyond survival to prevention, avoidance of antibiotic overuse, and reduction of long-term mortality and cardiovascular complications [12,133,212,302].
From prolonged empiricism to shorter, targeted therapy
Treatment of community-acquired pneumonia (CAP) has progressively moved from prolonged, broadly empirical antimicrobial therapy toward risk-adapted selection, early reassessment, oral step-down, and shorter courses. In children hospitalised with severe CAP, the PediCAP factorial randomised trial directly tested the WHO recommendation of 5 days of intravenous antibiotics, evaluating whether clinically stable children without complicating factors could safely transition to oral treatment and receive a shorter total course [472]. The trial enrolled children aged 2 months to 6 years across 13 hospitals in five sub-Saharan African countries and compared oral amoxicillin with amoxicillin–clavulanate after intravenous treatment; its design represents a major shift from maintaining intravenous therapy by default to testing structured de-escalation [472].
Antimicrobial choice has also become increasingly individualized. In adults with cancer, the INSPIRE trials evaluated computerized prescribing prompts that supplied patient- and pathogen-specific multidrug-resistant organism (MDRO) risk estimates, addressing the routine use of extended-spectrum antibiotics despite a generally low prevalence of MDROs [133]B2b. A separate pragmatic randomised trial used electronic health-record interpretation of low procalcitonin or a positive respiratory-virus test to estimate the post-test probability of bacterial pneumonia and guide antibiotic decisions, responding to evidence that test results alone often do not change prescribing [306]A1b. In suspected CAP requiring intensive care, the STOP-Vanc randomised trial assessed MRSA nasal polymerase-chain-reaction testing as a tool for reducing unnecessary empiric vancomycin exposure [134]A1b. Together, these studies illustrate the evolution from “cover first and continue” to explicit probability-based initiation, narrowing, and discontinuation of therapy [133]B2b[134]A1b[306]A1b.
Expanding pathogen-directed treatment
The treatment history of CAP has increasingly incorporated rapid recognition of pathogens that require specific therapy. For Legionella pneumonia, a multicentre cohort evaluated time to effective Legionella-directed treatment—defined as a macrolide, fluoroquinolone, or doxycycline—and examined its relationship with mortality, including the excess risk observed in immunocompromised patients [461]. Chlamydia psittaci remains diagnostically difficult because of nonspecific manifestations; targeted next-generation sequencing of respiratory specimens was evaluated as a means of improving diagnosis and antimicrobial stewardship, with particular attention to empirical quinolone treatment [485]C. In a separate cohort, CT patterns and quantitative CT severity scores were assessed for their association with intensive-care admission in C. psittaci pneumonia, potentially supporting earlier escalation and targeted management [278]B2b. A retrospective risk model using creatine kinase and direct bilirubin was proposed specifically to identify patients already presenting with severe C. psittaci pneumonia, although it was hypothesis-generating and not a validated progression model [210]B2b.
Complicated pediatric pneumonia has likewise driven a move beyond antibiotics alone. In necrotizing pneumonia, a retrospective cohort found that 10.55% of 398 children required surgery after failure of medical therapy; Mycoplasma pneumoniae was the most frequently identified pathogen, and the cohort described substantial preoperative morbidity and postoperative resource use [519]. A pediatric pyopneumothorax caused by Prevotella oris highlighted the role of metagenomic next-generation sequencing in diagnosing unusual anaerobic infection and informing therapeutic management [465]. These reports support escalation to drainage or surgery when infection is destructive or inadequately controlled medically, while emphasizing that the evidence remains observational or case-based [465][519].
Adjunctive and host-directed approaches
Treatment evolution has also included adjunctive therapies intended to modify inflammation, physiology, or complications. In hospitalised CAP, the ACCESS randomised trial found that adding clarithromycin to standard care produced earlier symptom resolution during the first 72 hours and reduced progression to respiratory failure and secondary sepsis; molecular analyses identified treatment-associated changes in gene-expression and immune-response pathways [303]A1b. In a small randomised trial of 30 patients, infrared photobiomodulation added to antibiotics and physiotherapy was evaluated over 7 days for pulmonary-function and haematological outcomes, reflecting experimental interest in non-antimicrobial supportive treatment [70]A1b. A pilot study in 84 enterally fed patients with severe pneumonia tested 3.50 g versus 8.75 g daily of omega-3 fatty acids for 7 days; supplementation was associated with shorter mechanical-ventilation duration and lower hospital costs, although the anticipated anti-inflammatory mechanism was not demonstrated [477]C.
COVID-19-associated pneumonia accelerated adaptive-platform evaluation of adjunctive therapies. REMAP-CAP evaluated ivermectin in critically and noncritically ill hospitalised patients with COVID-19, using respiratory- and cardiovascular-organ-support-free days as the primary ordinal outcome [300]A1b. The same platform evaluated intermediate-dose versus standard low-dose heparin thromboprophylaxis in critically ill patients, with treatment continued for up to 14 days and organ-support-free days through day 21 as the primary outcome [304]A1b. These studies exemplify the modern preference for platform trials that rapidly assess pharmacologic and supportive interventions in clinically defined CAP populations [300]A1b[304]A1b.
Prevention and longer-term treatment goals
Prevention has become part of the broader CAP treatment pathway. In adults aged 65 years or older, a cluster-randomised crossover study compared adjuvanted with high-dose inactivated influenza vaccines against PCR-confirmed influenza, reflecting efforts to prevent influenza-associated pneumonia in older adults [302]A1b. For children with mild Mycoplasma pneumoniae pneumonia, a double-blind trial evaluated Lianhua Qingke tablets as a 7-day adjunct to conventional therapy in the context of increasing macrolide resistance [145]A1b. Finally, treatment success is now judged beyond discharge: the Long-term Pneumonia Mortality Index was derived and externally validated to estimate 1-year mortality after CAP, reinforcing the need to incorporate rehabilitation, comorbidity management, and follow-up into modern care [12]B2b. Prospective follow-up has also examined cardiovascular events after pneumonia, while the presence of Frank’s sign was studied as a possible marker of cardiovascular risk [212]B2b.
| Treatment phase or objective | Contemporary approach | Supporting evidence |
|---|---|---|
| Initial antibiotic strategy | Use clinical probability, biomarkers, respiratory-virus results, and individualized MDRO risk to avoid unnecessary broad-spectrum therapy | [133]B2b, [306]A1b |
| De-escalation | Test MRSA nasal PCR to reduce empiric vancomycin exposure; transition children from intravenous to oral therapy | [134]A1b, [472] |
| Duration | Investigate courses shorter than the historical default, including the WHO 5-day intravenous recommendation in severe pediatric CAP | [472] |
| Pathogen-directed therapy | Prioritize timely Legionella-active therapy and targeted treatment for C. psittaci or unusual anaerobes | [461], [465], [485]C |
| Complicated disease | Escalate to procedural or surgical management when necrosis, pyopneumothorax, or failed medical therapy occurs | [465], [519] |
| Long-term care | Address influenza prevention, 1-year mortality, and cardiovascular sequelae after pneumonia | [12]B2b, [212]B2b, [302]A1b |
9. Respiratory Support, Procedures & Interventional Pulmonology (Specialty-Distinctive)
- ▸The supplied evidence supports selective bronchoscopy for refractory pediatric CAP, suspected plastic bronchitis, and lower-airway microbiologic sampling; it does not define routine bronchoscopy for uncomplicated CAP. [366][432][487][491]
- ▸Plastic casts were identified in **100/761 (13.1%)** children with CAP who underwent bronchoscopy in a 2026 retrospective study. [487]
- ▸The 2026 IDSA/PIDS updates address thoracostomy-tube size and chest-tube fibrinolysis versus mechanical debridement for children older than **3 months**, using GRADE methodology. [358][359]
- ▸In a pediatric necrotizing-pneumonia cohort, **42/398 (10.55%)** children required surgery after failed medical therapy. [519]
- ▸The supplied references do not provide comparative thresholds for oxygen therapy, high-flow oxygen, noninvasive ventilation, or invasive mechanical ventilation. [243][358][359]
Scope and escalation
The available updated evidence is concentrated on bronchoscopy, lower-airway sampling, pleural drainage, fibrinolysis, and surgery rather than on comparative oxygen-delivery or invasive-ventilation strategies. In children with community-acquired pneumonia (CAP), these interventions are most relevant when disease is severe, complicated, diagnostically uncertain, or unresponsive to routine treatment. Bronchoscopy-based cohorts should not be interpreted as representative of all CAP because they selectively included children for whom bronchoscopy was clinically indicated. [366]B2b[432][488][491]
Respiratory support and procedural assessment
Respiratory support should be individualized to physiologic severity and monitored alongside work of breathing, oxygenation, ventilation, and hemodynamic status; however, the supplied references do not provide comparative thresholds for conventional oxygen, high-flow nasal oxygen, noninvasive ventilation, or invasive mechanical ventilation. In solid-organ transplant recipients hospitalized with CAP, ICU admission and mortality were specifically evaluated outcomes, underscoring the need for early severity assessment in immunocompromised patients, although the supplied abstract does not report procedure-specific respiratory-support results. [243]B2b
Bronchoscopy is a targeted diagnostic and therapeutic procedure rather than a routine test for uncomplicated CAP. In a 2026 pediatric cohort, 761 children with CAP underwent bronchoscopy; bronchoscopy identified plastic casts in 100/761 (13.1%), defining a plastic-bronchitis group for risk-factor analysis. [487]C Other pediatric investigations used bronchoalveolar lavage (BAL) for pathogen detection and clinical phenotyping, including a 305-patient cohort in which BAL microbiology, inflammatory characteristics, and clinical findings were integrated by unsupervised clustering to identify mixed-infection phenotypes. [432] A severe-CAP study similarly evaluated bronchoalveolar-lavage nucleic-acid testing for 19 pathogens in children aged 1 month to 14 years. [488]
In infants with CAP and cytomegalovirus DNA detected in BAL fluid, flexible bronchoscopy was performed when routine CAP treatment was ineffective; the study included 124 infants aged 1–11 months and distinguished recent infection from CMV replication using serologic results. [491] A multicenter pediatric study of 198 children indicated for fiberoptic bronchoscopy compared nasopharyngeal aspirates with BAL fluid using targeted next-generation sequencing and 16S rRNA sequencing; it reported high consistency between the first pathogen detected in upper- and lower-airway samples, but lower-airway microbiota varied across pathogen and outcome groups. [366]B2b
For adults, a prospective controlled comparison enrolled 181 patients with clinically diagnosed CAP and compared targeted next-generation sequencing in sputum with BAL fluid, addressing the practical situation in which bronchoscopy is not tolerated or BAL cannot be obtained promptly. [368]B2b In hematologic-malignancy patients hospitalized with CAP, a retrospective cohort of 524 patients included individuals who underwent bronchoscopy within 24 hours of admission for etiologic assessment. [489] Bedside fiberoptic bronchoscopy and sputum obtained through the procedure supported metagenomic diagnosis of Chlamydia psittaci pneumonia in a patient with motor-neuron disease; doxycycline was then administered. [364]C4 A 40-patient mNGS-confirmed C. psittaci series also characterized bronchoscopic findings and features associated with severe CAP. [527]C
Plastic bronchitis and airway obstruction
Plastic bronchitis is a bronchoscopic diagnosis defined in the cited pediatric study by visualization of airway casts. Among 761 bronchoscoped children with CAP, 100 had plastic bronchitis; the study analyzed pathogen distribution, clinical manifestations, laboratory findings, and prognostic indicators to identify associated risk factors. [487]C These data support bronchoscopy when airway obstruction, lobar collapse, refractory hypoxemia, or failure to improve raises concern for obstructing casts, but they do not establish universal indications or a standardized cast-clearance technique. [487]C
Pleural infection: drainage and tube selection
The 2026 Infectious Diseases Society of America/Pediatric Infectious Diseases Society guideline update addresses parapneumonic effusion and empyema in infants and children older than 3 months. Its thoracostomy-tube-size guideline uses systematic-literature-review evidence and GRADE methodology to make recommendations on the appropriate tube size for pleural drainage. [358]A1c The supplied abstract does not state the preferred size or a size-specific threshold; therefore, tube selection should follow the full guideline recommendation and be individualized to patient size, fluid characteristics, access, and local expertise. [358]A1c
A companion guideline evaluates pleural-fluid drainage with chest-tube fibrinolysis versus mechanical surgical debridement, also using systematic reviews and GRADE. [359]A1c The abstract confirms that this is the relevant decision framework but does not provide the recommendation direction or certainty grade; no stronger conclusion should be inferred from the citation alone. [359]A1c
Necrotizing and complicated pneumonia: surgery and follow-up
In a retrospective cohort of 398 children with necrotizing pneumonia diagnosed from 2017 through June 2025, 42 (10.55%) required surgery because medical therapy failed. Mycoplasma pneumoniae was the most frequently identified pathogen, present in 52.50% of the surgical cohort; preoperative complications occurred in 83.33%. Mean operative time was 140.71 ± 49.84 minutes, blood loss 119.79 ± 49.97 mL, chest-tube duration 7.83 ± 3.05 days, and postoperative hospitalization 14.74 ± 3.53 days. [519] These retrospective data describe outcomes after selected rescue surgery and do not establish surgery as first-line therapy. [519]
Long-term consequences matter when deciding whether invasive management has achieved recovery. A multicenter retrospective study of 123 children hospitalized with necrotizing pneumonia or parapneumonic effusion evaluated follow-up spirometry, radiologic sequelae, treatment modalities, and lung function. [486] Complicated CAP caused by group A streptococcus was also prominent during a 2022–2023 outbreak-season series: GAS accounted for 22/91 (24.2%) complicated-CAP cases. [490] Etiology and complication burden should therefore be incorporated into follow-up planning, although these studies do not define a universal spirometry schedule. [486][490]
Practical synthesis
Use bronchoscopy selectively for refractory disease, suspected casts or airway obstruction, and targeted lower-airway microbiology; use pleural drainage for clinically significant parapneumonic infection according to pediatric guideline criteria; and reserve surgical debridement or resection for selected patients with inadequate response to medical and drainage strategies. [358]A1c[359]A1c[487]C[519] Evidence from retrospective cohorts is hypothesis-generating and should not replace multidisciplinary assessment by respiratory, infectious-disease, critical-care, interventional, and surgical teams. [243]B2b[486][519]
| Clinical problem | Procedure or evidence source | Key reported data | Evidence limitations |
|---|---|---|---|
| Suspected plastic bronchitis | Diagnostic/therapeutic bronchoscopy | 100/761 children had casts | Retrospective, bronchoscopy-selected cohort [487]C |
| Refractory or severe pediatric CAP | BAL and bronchoscopy | BAL used for mixed-infection phenotyping and pathogen testing | Indication was selective; no universal timing standard [432][488][491] |
| Parapneumonic effusion/empyema | Thoracostomy drainage | 2026 guideline addresses tube size | Supplied abstract does not state the preferred size [358]A1c |
| Pleural infection requiring escalation | Chest-tube fibrinolysis versus surgery | 2026 guideline compares drainage with fibrinolysis against mechanical debridement | Supplied abstract does not state recommendation direction [359]A1c |
| Necrotizing pneumonia failing medical therapy | Surgery | 42/398 children required surgery | Retrospective rescue-surgery data; not first-line evidence [519] |
10. Complications
- ▸Respiratory failure is a central CAP complication; it occurred in 15%–20% of hospitalized children with MPP in the cohort used to develop a prediction model. [511]
- ▸In adults with CAP and T2DM, septic shock, respiratory failure, and mortality were higher than in adults without diabetes: 6.4% versus 3.5%, 22.7% versus 16.5%, and 8.4% versus 4.3%, respectively. [456]
- ▸Pediatric necrotizing pneumonia may require surgery after failed medical therapy; 10.55% of 398 children underwent intervention in one cohort. [519]
- ▸Empyema and pyopneumothorax are severe pleural complications, including rare anaerobic infection caused by Prevotella oris. [465]
- ▸Cardiovascular complications after pneumonia include atrial fibrillation/flutter and myocardial infarction. [212]
- ▸Mycoplasma pneumoniae can cause extrapulmonary mucositis, including MIRM. [222]
- ▸Pathogen co-detection, Staphylococcus aureus–viral coinfection, adenovirus infection, and Chlamydia pneumoniae infection have each been studied in relation to severe CAP or adverse outcomes. [467] [444] [492] [497]
Community-acquired pneumonia (CAP) may be complicated by respiratory failure, septic shock, cardiovascular events, pleural infection, pneumothorax, pulmonary necrosis, extrapulmonary immune-mediated disease, and death. The likelihood and pattern of complications vary according to age, pathogen, comorbidity, coinfection, and initial disease severity. Evidence summarized below is derived largely from hospitalized cohorts and therefore may overrepresent severe CAP.
Respiratory failure and death
Respiratory failure is a major complication of CAP and is particularly prominent in severe pediatric Mycoplasma pneumoniae pneumonia (MPP), in which it occurred in 15%–20% of hospitalized cases in the cohort informing a clinical prediction model. [511] In that study, 62 of 305 children developed respiratory failure, although the abstract does not provide the complete independent predictor set or model performance. [511] A separate external-validation study evaluated a machine-learning model for identifying hospitalized adults with CAP and acute hypoxemic respiratory failure who were at low risk of subsequent respiratory deterioration or death using data available within the first 6 hours of admission. [539]
Among adults with severe CAP admitted to intensive care units (ICUs), mortality remains substantial. A multicenter prospective Chinese study enrolled 170 patients with severe CAP, including 111 ICU patients and 59 ward patients, to examine clinical characteristics and mortality-associated factors; the available abstract confirms the high-risk ICU population but does not report the complete mortality estimate or final predictor list. [495] In a real-world study of adults treated with omadacycline, clinical failure, 30-day mortality, microbial eradication, and adverse events were assessed, with multivariable logistic regression, Kaplan–Meier analysis, and Cox regression used to identify prognostic factors. [514]
Type 2 diabetes mellitus (T2DM) is associated with greater complication risk in CAP. In a retrospective cohort of 2,471 patients, 559 (22.6%) had T2DM; septic shock occurred in 6.4% versus 3.5%, respiratory failure in 22.7% versus 16.5%, and mortality in 8.4% versus 4.3% among patients with and without diabetes, respectively. [456] These findings support diabetes as an important risk marker for respiratory and circulatory complications, although the observational design cannot establish causality. [456]
Cardiovascular complications
Pneumonia can be followed by clinically important cardiovascular events. A prospective single-center cohort of adults hospitalized with CAP assessed an 18-month composite of cardiovascular outcomes, including new-onset atrial fibrillation or flutter and myocardial infarction, in relation to Frank’s sign, an acquired diagonal earlobe crease associated with underlying cardiovascular disease. [212]B2b The study was designed to determine whether this physical finding identifies patients at increased risk after pneumonia; the supplied abstract does not provide the effect estimate or event frequency. [212]B2b
Pleural, air-leak, and necrotizing complications
Empyema and pyopneumothorax are severe pleural complications of pediatric CAP. [465] A case report described bilateral pneumonia and pyopneumothorax caused by the anaerobe Prevotella oris in a 10-year-old boy after tooth extraction, illustrating the diagnostic challenge of an uncommon pathogen and the potential relevance of an antecedent dental event. [465] The report used metagenomic next-generation sequencing to support diagnosis. [465]
Pneumothorax has also been evaluated in adults hospitalized with pneumonia. A retrospective TriNetX cohort compared fluoroquinolone exposure with non-fluoroquinolone CAP antibiotics and assessed pneumothorax within 60 days using adjusted Cox regression after excluding patients with connective-tissue disorders. [84]B2b The supplied abstract does not state the adjusted hazard ratio; therefore, it does not establish whether fluoroquinolone treatment causes or prevents pneumothorax. [84]B2b
Necrotizing pneumonia (NP) represents destructive parenchymal disease and may require surgery when medical therapy fails. In a retrospective pediatric cohort of 398 children with NP, 42 (10.55%) underwent surgical intervention for failed medical therapy, pathogens were identified in 95.2%, and Mycoplasma pneumoniae was the most common identified pathogen (52.50%). [519] Preoperative complications were present in 83.33% of surgical patients; reported perioperative measures included mean operative time of 140.71 ± 49.84 minutes, blood loss of 119.79 ± 49.97 mL, chest-tube duration of 7.83 ± 3.05 days, and postoperative hospitalization of 14.74 ± 3.53 days. [519] The study also compared parenchymal resection with non-resection approaches, but the supplied abstract does not provide the complete long-term outcome data. [519]
Pathogen-associated and extrapulmonary complications
Respiratory pathogen co-detection may worsen pediatric MPP. In a cohort of 3,081 children with confirmed MPP, 1,908 had respiratory pathogen co-detection and 1,173 had MPP alone; the study compared severe MPP, specific complications, healthcare utilization, and costs according to co-detection pattern. [467] The abstract establishes the scale and outcomes assessed but does not provide the complete adjusted estimates. [467]
Mycoplasma pneumoniae may cause mucocutaneous disease in addition to pneumonia. Mycoplasma pneumoniae-induced rash and mucositis (MIRM) is characterized particularly by prominent mucositis; a systematic review and case series evaluated diagnostic criteria, clinical features, and outcomes of oral mucositis in confirmed infection. [222]B2a The available abstract does not provide pooled frequencies or outcome estimates. [222]B2a
Other pediatric pathogens may produce severe CAP phenotypes. A 2021–2023 Shanghai study evaluated adenovirus type, viral load, and severe CAP outcomes in hospitalized children using droplet digital PCR and multivariable analysis. [492] A separate post-pandemic study enrolled 121 children with Chlamydia pneumoniae pneumonia through 2025 and examined factors associated with severe disease under the 2024 revised pediatric CAP criteria. [497]C A large Northeast China study of 17,505 hospitalized children with MPP assessed adverse outcomes and associated factors across 2018–2023, including changes before, during, and after the COVID-19 pandemic. [451]
Coinfection, inflammation, and outcome assessment
Staphylococcus aureus–viral coinfection is associated with complex pathogenesis and potentially poor prognosis; a genomic and clinical study specifically evaluated its clinical characteristics and prognostic impact in CAP. [444] The supplied abstract does not provide the final effect estimates. Metabolomic profiling has also been used to compare hospitalized CAP with COVID-19, using age-, sex-, and Charlson Comorbidity Index-matched cohorts and serum collected at admission. [536] These findings are investigational and should not be used alone to define a CAP complication.
Influenza vaccination status has been examined in adults hospitalized with CAP in Saudi Arabia, with outcomes including laboratory-confirmed influenza, ICU admission, in-hospital mortality, and length of stay. [535] Because the supplied abstract does not report the adjusted results, vaccination status should not be interpreted from this study alone as a proven modifier of complications. Adjunct systemic corticosteroids in children with CAP have been evaluated in a systematic review and meta-analysis of randomized trials, focusing on benefits and harms when corticosteroids are added to standard care. [537] The abstract does not provide the pooled estimates; corticosteroid use should therefore be individualized rather than inferred from the review summary alone. [537]
| Complication or risk domain | Evidence | Reference |
|---|---|---|
| Respiratory failure | 15%–20% reported among hospitalized pediatric MPP cases; 62/305 children in a prediction-model cohort | [511] |
| Septic shock and mortality | Higher with T2DM: septic shock 6.4% vs 3.5%; mortality 8.4% vs 4.3% | [456] |
| Cardiovascular events | New atrial fibrillation/flutter and myocardial infarction assessed during 18-month follow-up | [212]B2b |
| Pleural infection and air leak | Empyema and pyopneumothorax, including Prevotella oris-associated disease | [465] |
| Necrotizing pneumonia | 42/398 children required surgery after failed medical therapy; 83.33% had preoperative complications | [519] |
| Extrapulmonary disease | MIRM and oral mucositis associated with M. pneumoniae | [222]B2a |
| Coinfection-related severity | Respiratory pathogen co-detection in pediatric MPP and S. aureus–viral coinfection evaluated for adverse outcomes | [467] [444] |
11. Prognosis and Natural History
- ▸CAP prognosis ranges from uncomplicated recovery to respiratory failure, septic shock, ICU admission, and death; risk is heterogeneous and requires dynamic reassessment. [449][514][517]
- ▸Approximately one in five hospitalized patients in the PROGRESS context required ICU admission, and delayed ICU admission was associated with higher mortality risk. [449]
- ▸Frailty, immune-inflammatory dysregulation, viral pneumonia, Pseudomonas burden, pulmonary artery diameter, and social determinants are emerging prognostic domains. [518][520][521][522][543]
- ▸In children, admission inflammatory phenotypes may predict severe CAP and pediatric ARDS, while necrotizing pneumonia signals a complicated course with frequent complications and occasional need for surgery. [425][519]
- ▸Transcriptomic, biomarker, imaging, microbiome, and machine-learning tools are promising but are not yet substitutes for validated severity assessment and repeated clinical examination. [449][517][521][543]
Overview
Community-acquired pneumonia (CAP) has a heterogeneous course, ranging from uncomplicated recovery to respiratory failure, septic shock, intensive-care admission, and death. Prognosis is determined by initial disease severity, age, frailty, comorbidity, pathogen, inflammatory response, organ dysfunction, and the speed with which deterioration is recognized and treated. Recent evidence remains predominantly observational and is concentrated in hospitalized adults, critically ill patients, older adults, and children; therefore, reported associations should not be interpreted as universal mortality estimates for all patients with CAP. [449][514][517][520][522]
In the prospective PROGRESS cohort, approximately one in five hospitalized patients with CAP required intensive-care admission, and delayed ICU admission was associated with increased mortality risk. A five-gene blood transcriptomic signature was evaluated as a potential method for identifying patients at risk of organ dysfunction and death beyond conventional clinical scores, but the supplied evidence does not provide sufficient performance estimates to replace established bedside assessment. [449]
Predictors of severe disease and death in adults
Among critically ill patients with severe CAP, immune-inflammatory dysregulation is an important source of prognostic heterogeneity. A retrospective cohort of 223 patients developed an immune-inflammatory dysregulation score using six biomarkers and evaluated its ability to stratify 28-day all-cause mortality; this approach is potentially complementary to conventional scores, which mainly reflect comorbidity and organ dysfunction. External validation and prospective clinical utility remain necessary. [517]
In critically ill patients with viral pneumonia, machine-learning models were developed to predict 28-day mortality and new invasive mechanical ventilation using MIMIC-IV data, with external validation of the mortality model in another hospital cohort. Blood urea nitrogen, albumin, and the blood-urea-nitrogen-to-albumin ratio were specifically investigated as prognostic variables. These models remain retrospective and should support, rather than replace, clinical reassessment. [543]
A larger pulmonary artery diameter measured on CT pulmonary angiography was associated with mortality in a retrospective study of 243 pneumonia cases. Measurement methods had strong inter-method agreement, but the finding requires validation before pulmonary artery diameter can be used routinely alongside CURB-65 or the Pneumonia Severity Index. [521]
Frailty is particularly relevant in older adults. In a cohort of patients aged ≥60 years, a locally derived electronic-medical-record frailty index was evaluated for associations with septic shock and all-cause mortality. The study supports frailty assessment as a potential addition to conventional risk stratification, although the index was locally constructed and may not generalize across healthcare systems. [522]
Sociodemographic factors may also influence outcomes. A multicentre Ontario cohort examined ICU admission or in-hospital mortality across adults aged 18–39, 40–64, and ≥65 years, while adjusting for clinical and comorbidity-related factors. The study was designed to determine whether associations differed by age, indicating that prognosis should be interpreted within both clinical and social contexts rather than by age alone. [520]
Pathogen- and microbiome-associated prognosis
Pseudomonas aeruginosa is associated with poor outcomes in severe CAP. In a multicentre retrospective study of 130 patients with P. aeruginosa severe CAP, metagenomic quantification of pathogen burden was linked to lung microbiome alterations, including microbiome collapse, and clinical outcomes. The findings suggest that quantitative microbial burden may provide prognostic information beyond simple pathogen detection, but the observational design prevents causal inference. [518]
Pneumococcal epidemiology continues to change in the post-conjugate-vaccine era because serotype replacement alters the distribution of pneumococcal CAP. A Swiss prospective cohort evaluated serotype distribution, vaccine coverage, and the diagnostic yield of a serotype-specific urinary antigen assay; these data are relevant to pathogen attribution and prevention but do not by themselves establish individual mortality risk. [541]C
Rapid pathogen detection may affect the clinical trajectory indirectly by enabling earlier targeted treatment. In children with CAP, targeted next-generation sequencing was evaluated in a retrospective single-centre cohort of 838 patients for pathogen detection and clinical utility. In severe ICU CAP, rapid multiplex PCR testing was examined in 288 adults, specifically assessing antibiotic escalation after test results; escalation may reflect recognition of resistant or severe infection rather than treatment failure alone. [452][542]
Children and complicated pneumonia
In children younger than 5 years hospitalized with CAP, admission inflammatory biomarkers—including C-reactive protein, procalcitonin, neutrophil-to-lymphocyte ratio, platelet-to-lymphocyte ratio, lymphocyte-to-monocyte ratio, systemic immune-inflammation index, and systemic inflammation response index—were used to define inflammatory phenotypes associated with severe CAP and in-hospital pediatric acute respiratory distress syndrome. These markers may assist early risk stratification, but biomarker thresholds and external validity require confirmation. [425]
Necrotizing pneumonia represents a complicated course with substantial morbidity. In a retrospective cohort of 398 children, 42 (10.55%) required surgery after failure of medical therapy; Mycoplasma pneumoniae was the most frequently identified pathogen, accounting for 52.50% of cases, and 83.33% had preoperative complications. Surgical outcomes included prolonged chest-tube use and postoperative hospitalization, while comparisons between parenchymal resection and non-resection procedures indicated differences in recovery; the supplied abstract does not provide complete long-term outcome estimates. [519]
Recovery also depends on functional rehabilitation. A randomized trial enrolled 170 children recovering from pneumonia, of whom 154 completed the study, to compare a game-based respiratory rehabilitation intervention with conventional rehabilitation. The study assessed respiratory function, exercise capacity, and adherence; the available abstract establishes the trial population and outcomes but does not provide numerical results sufficient to quantify long-term benefit. [540]
Treatment response and follow-up implications
In older hospitalized adults, a prospective non-randomized study compared standard care with or without adjunctive Chaihu Guizhi Tang plus Maxing Shigan Tang, using day-7 clinical cure, 72-hour response, and time to clinical stabilization as outcomes. Because treatment was non-randomized and open-label, any apparent improvement should be considered associative rather than definitive evidence that the adjunct changes the natural history of CAP. [513]
Real-world omadacycline studies evaluated clinical failure, microbiological eradication, adverse events, and 30-day mortality in adults, including older non-ICU patients, while examining factors associated with poor outcome. A separate pediatric cohort assessed intravenous omadacycline for macrolide-unresponsive Mycoplasma pneumoniae pneumonia in children aged 8–16 years, using time to defervescence and radiological improvement as outcomes. These studies inform treatment response in selected populations but should not be generalized to untreated CAP or all age groups. [514][424][475]C
Influenza vaccination status was examined in adults hospitalized with CAP in Saudi Arabia, with ICU admission, in-hospital mortality, and length of stay as outcomes. Because the study was retrospective, vaccination status may be confounded by baseline health, access to care, and preventive-health behavior. [535]
Prognostic interpretation
Clinical deterioration should be assessed dynamically, especially during the first days of hospitalization, because delayed recognition of organ dysfunction may worsen outcomes. Current evidence supports combining validated severity assessment with repeated evaluation of respiratory status, hemodynamics, renal function, inflammatory response, frailty, pathogen risk, and treatment response. Novel transcriptomic, biomarker, imaging, microbiome, and machine-learning tools are promising but require prospective validation before routine replacement of established clinical judgment. [449][517][521][522][543]
A retrospective study of doxycycline versus azithromycin combined with a beta-lactam in ICU patients with severe CAP evaluated comparative outcomes, but the supplied abstract does not provide outcome estimates; conclusions should therefore remain limited to the study question. Similarly, data describing pathogen trends in ventilator-associated and non-ventilator hospital-acquired pneumonia should not be extrapolated to community-acquired disease. [453][455]
| Domain | Potential prognostic relevance | Evidence |
|---|---|---|
| Early clinical trajectory | ICU need, organ dysfunction, and mortality risk; delayed ICU admission is concerning | [449] |
| Inflammation and immune response | Severe CAP, PARDS, 28-day mortality, and immune dysregulation | [425][517] |
| Frailty and age | Septic shock and mortality risk in older adults | [522] |
| Pathogen burden | P. aeruginosa burden and microbiome collapse in severe CAP | [518] |
| Imaging | Pulmonary artery diameter associated with mortality | [521] |
| Functional recovery | Respiratory function, exercise capacity, and rehabilitation adherence in children | [540] |
12. Special Populations & Pregnancy
- ▸The supplied evidence does not provide pregnancy-specific CAP outcomes or antimicrobial recommendations. [540,514,523,424,425,475,450,515,542,524,519,451,452,454,465,459,461,471,544,545]
- ▸In children, examination findings, inflammatory biomarkers, molecular diagnostics, and age-specific epidemiology should be integrated rather than used in isolation. [523,425,451,452]
- ▸Macrolide-unresponsive Mycoplasma pneumoniae pneumonia in adolescents has only limited retrospective evidence for intravenous omadacycline. [475]
- ▸Older-adult omadacycline evidence is observational and includes comparisons with tigecycline, moxifloxacin, and cefoperazone–sulbactam. [424,514]
- ▸The CALLY index is investigational and should not replace PSI, CURB-65, or clinical judgment. [545]
- ▸Necrotizing pneumonia, pyopneumothorax, PARDS risk, Legionella infection, and severe atypical-pathogen disease warrant specialist-level assessment. [425,461,465,519,544]
Scope and evidence limitations
The supplied 2026 evidence base is predominantly observational and pediatric or geriatric; it does not provide pregnancy-specific CAP outcomes, validated pregnancy-adapted severity thresholds, or pregnancy-specific antimicrobial recommendations. Therefore, management during pregnancy should not be inferred from the pediatric or elderly cohorts summarized below, and treatment decisions require pregnancy-specific guideline and obstetric–infectious-disease review. [540][514][523][424][425][475]C[450][515][542][524]C[519][451][452][454][465][459][461][471][544][545]
Children and adolescents
Children require age-appropriate assessment because the reliability of individual examination findings is variable. In a prospective multicenter study of youths aged 3 months to 17 years evaluated in seven US emergency departments, investigators assessed interrater reliability of physical-examination findings among patients diagnosed with CAP; the study was specifically designed to inform outpatient diagnosis, for which guidelines emphasize clinical examination rather than routine imaging. [523] The supplied abstract does not provide the individual reliability estimates, so no single examination sign should be treated as definitive on the basis of this reference alone. [523]
Admission inflammatory markers may assist risk stratification in young children but should not replace clinical assessment. A prospective cohort enrolled children aged <5 years hospitalized with CAP and evaluated neutrophil-to-lymphocyte ratio, platelet-to-lymphocyte ratio, lymphocyte-to-monocyte ratio, systemic immune-inflammation index, systemic inflammation response index, C-reactive protein, and procalcitonin as potential inflammatory phenotypes associated with severe CAP and in-hospital pediatric acute respiratory distress syndrome (PARDS). [425] A separate retrospective nomogram study included 1,486 hospitalized children and evaluated etiologic, epidemiologic, and clinical risk factors for severe CAP, with Streptococcus pneumoniae and Haemophilus influenzae among the leading detected bacteria. [471]
Etiology varies with age, geography, season, and surveillance period. In a large Chinese cohort of 17,505 hospitalized children with Mycoplasma pneumoniae pneumonia (MPP), spanning 2018–2023 and ages 28 days to 14 years, the highest annual case count occurred in 2023; the study examined adverse outcomes and associated factors before, during, and after the COVID-19 period. [451] Another Chinese cohort assessed pathogen patterns in hospitalized children from 2018–2023 and reported changes in bacterial and respiratory-viral detection in association with pandemic-era nonpharmaceutical interventions. [459] Targeted next-generation sequencing was evaluated in 838 children with CAP admitted between 2023 and 2025 as a potential tool for pathogen detection and clinical decision-making. [452]
Macrolide-unresponsive MPP remains a special therapeutic problem. In a retrospective cohort of 64 hospitalized children aged 8–16 years with persistent fever above 38.0°C after 72 hours of azithromycin, 18 received intravenous omadacycline and 46 continued azithromycin; omadacycline was administered using body-surface-area scaling from the adult regimen, with time to defervescence, 72-hour defervescence, radiologic improvement, corticosteroid use, and short-term safety evaluated. [475]C The nonrandomized design and small omadacycline group limit conclusions regarding comparative efficacy or routine pediatric use. [475]C In a separate randomized trial of children recovering from pneumonia, 170 participants were allocated to fun-game respiratory rehabilitation or conventional rehabilitation, and 154 completed the study, with respiratory function, exercise capacity, and compliance assessed. [540]
Complicated pediatric disease requires specialist care. Among 398 children with necrotizing pneumonia, 42 (10.55%) required surgery after failure of medical therapy; pathogens were identified in 95.2%, with M. pneumoniae reported as the most common organism in that cohort. [519] Pediatric pyopneumothorax caused by the anaerobe Prevotella oris was described after recent tooth extraction, emphasizing that unusual anaerobic infection may occur in severe pleuropulmonary disease and may require advanced microbiologic diagnosis. [465] Routine blood cultures have low clinical utility in uncomplicated pediatric CAP, and a large hospital-database study evaluated variation in testing among children without complicated infection, intensive-care admission, complex chronic disease, or prolonged hospitalization. [524]C
Older adults
Older adults have increased clinical complexity and may require individualized assessment of severity, organ function, drug interactions, and treatment tolerance. A real-world study evaluated intravenous omadacycline in non-ICU hospitalized elderly patients with CAP against tigecycline, moxifloxacin, and cefoperazone–sulbactam using inverse-probability-of-treatment weighting; analyses included age subgroups <75 versus ≥75 years, clinical response, hospitalization, antibiotic duration, microbiologic clearance, and adverse events. [424] A separate real-world study assessed omadacycline-treated Chinese adults with CAP, examining clinical success, 30-day mortality, microbial eradication, adverse events, and predictors of clinical failure and mortality. [514] These observational studies support comparative evaluation in selected patients but do not establish superiority or a universal elderly treatment pathway. [424][514]
The C-reactive-protein–albumin–lymphocyte (CALLY) index was prospectively evaluated in 190 hospitalized, elderly-predominant patients with CAP and compared with PSI and CURB-65 for in-hospital mortality discrimination. [545] It may be considered an investigational adjunct to established severity assessment, not a replacement for validated scores or clinical judgment. [545]
Immunocompromised patients and severe disease
Immunocompromised patients with Legionella pneumonia may be particularly vulnerable to delayed effective therapy. A multicenter retrospective cohort of adults with laboratory-confirmed community-acquired Legionella pneumonia evaluated time from presentation to effective treatment with a macrolide, fluoroquinolone, or doxycycline and examined mortality differences associated with immune status. [461] Severe CAP requiring ICU admission may also be affected by rapid molecular testing practices: in 288 ICU patients, investigators evaluated antibiotic escalation after BioFire FilmArray Pneumonia Panel results, highlighting that rapid diagnostics can influence broadening as well as de-escalation. [542]
Certain atypical pathogens warrant heightened suspicion in severe presentations. Among 231 hospitalized patients with Chlamydia psittaci pneumonia, including 84 severe cases, machine-learning models were developed to classify severe versus nonsevere disease using clinical data. [544] In an Italian multicenter cohort of 176 adults hospitalized outside the ICU with CAP and PaO₂/FiO₂ <300, rapid molecular diagnostics were evaluated in relation to microbiology and hospital outcomes. [450] Human metapneumovirus is a globally circulating respiratory pathogen; a systematic review and meta-analysis assessed its prevalence, case-fatality rate, age-related severity, and seasonality across acute respiratory infection and CAP studies. [515]
Pregnancy
No supplied study reports pregnancy-specific CAP presentation, fetal outcomes, diagnostic thresholds, or antimicrobial safety comparisons. [540][514][523][424][425][475]C[450][515][542][524]C[519][451][452][454][465][459][461][471][544][545] Consequently, pregnancy should be managed as an evidence gap in this update rather than extrapolated from children, older adults, or nonpregnant adults. [540][424][461]
| Population or scenario | Evidence and practical implication |
|---|---|
| Children and adolescents | Use integrated clinical assessment; consider inflammatory markers and targeted molecular testing for selected hospitalized patients. [523][425][452] |
| Macrolide-unresponsive MPP, age 8–16 years | Small retrospective cohort evaluated IV omadacycline after fever persisted >72 hours on azithromycin; evidence is insufficient for routine use. [475]C |
| Older adults | Real-world studies evaluated omadacycline versus tigecycline, moxifloxacin, and cefoperazone–sulbactam, including <75 versus ≥75 years analyses. [424] |
| Immunocompromised adults with Legionella | Earlier effective Legionella-directed therapy and immune status were evaluated in relation to mortality. [461] |
| Pregnancy | No pregnancy-specific outcomes or treatment recommendations were reported in the supplied references. [540][514][424][461] |
13. Prevention, Screening & Surveillance
- ▸Use enhanced influenza-vaccine comparative evidence in adults aged **≥65 years**, but do not infer superiority of adjuvanted versus high-dose vaccine without the reported trial results. [302]
- ▸Prioritize vaccination assessment in CKD, HIV, cancer, and older adults, incorporating age, immune status, comorbidity, and duration of risk. [415][416][417]
- ▸Interpret pneumococcal vaccine effectiveness according to population, outcome, coverage setting, and follow-up duration; evidence regarding PPV23 protection beyond **5 years** remains a specific research focus. [505]
- ▸Use molecular and genomic surveillance for pneumococcus, hMPV, adenovirus, Mycoplasma pneumoniae, and Legionella to detect changing pathogen patterns and resistance. [515][525][492][516][507][501]
Vaccination and risk-based prevention
Vaccination remains the principal preventive strategy for pneumococcal and influenza-associated pneumonia, particularly in older adults and people with chronic disease. A 2026 cluster-randomized crossover study directly compared adjuvanted and high-dose inactivated influenza vaccines in adults aged ≥65 years across 65 Kaiser Permanente Northern California facilities during the 2023–2024 influenza season, with facilities switching vaccine allocation during the vaccination period; the prespecified outcome was PCR-confirmed influenza and the study was designed to estimate relative vaccine effectiveness. [302]A1b The available evidence therefore supports direct comparative evaluation of these two enhanced influenza-vaccine strategies, but the supplied abstract does not report the comparative effectiveness estimate and should not be interpreted as establishing superiority of either product. [302]A1b
Patients with non-dialysis chronic kidney disease (CKD) are an important prevention target because CKD is associated with increased infection risk, hospitalization, and mortality. [415]B2a A systematic review and meta-analysis searched MEDLINE, Embase, Scopus, and OVID through April 2025 for randomized and observational studies involving CKD stages 1–5 not receiving dialysis; vaccine efficacy was assessed using both immunologic outcomes, including seroconversion and antibody titres, and clinical outcomes. [415]B2a Vaccine counseling in CKD should therefore consider both immune response and protection against clinically important infection, while recognizing that the evidence base has historically focused more heavily on dialysis-dependent populations. [415]B2a
Pneumococcal vaccination is particularly relevant in older adults, although effectiveness estimates vary by population, outcome, vaccine, and follow-up duration. A multicenter matched case-control study in Eastern China evaluated 23-valent pneumococcal polysaccharide vaccine (PPV23) effectiveness against CAP hospitalization in adults aged ≥60 years, specifically addressing settings with low vaccination coverage and effectiveness more than 5 years after vaccination. [505] The study rationale identifies persistent controversy regarding PPV23 effectiveness against CAP and an evidence gap concerning duration of protection; the supplied abstract does not provide the final effectiveness estimates. [505] A separate retrospective cohort used causal-forest methods to examine whether the association between pneumococcal vaccination and severe CAP was uniform among hospitalized adults aged ≥65 years, emphasizing that vaccine-associated protection may be heterogeneous across elderly subgroups. [503]
People living with HIV remain at increased risk of CAP despite contemporary antiretroviral therapy. A nationwide longitudinal cohort of adults with HIV in care from 2010–2023 stratified CAP incidence by age, CD4 count, and duration of antiretroviral therapy, providing a framework for more individualized vaccination and prevention strategies. [417]B2b Prevention planning in HIV should therefore incorporate immune status and age rather than relying on either variable alone. [417]B2b
Targeted surveillance populations
Cancer patients with CAP may have substantial pneumococcal disease burden and higher mortality than the general population. [416]B2b In a prospective study at Memorial Sloan Kettering Cancer Center, 912 adults with CAP were evaluated using pneumococcal urinary antigen testing and a PCV15 serotype-specific urinary antigen detection assay; the median age was 67 years, and 94.6% were hospitalized. [416]B2b Serotype surveillance in immunocompromised populations can help determine whether vaccine-covered pneumococcal serotypes remain clinically important and can inform prevention priorities. [416]B2b
Pneumococcal disease also produces a burden extending beyond the acute admission. A nationwide Finnish registry cohort included adults with invasive pneumococcal disease, inpatient CAP, outpatient CAP, and matched controls; 40%–60% of affected patients had at least one predefined medical risk factor, while 34%–40% of patients without such factors were also represented in the cohort. [502] This supports surveillance strategies that include both recognized high-risk groups and apparently lower-risk adults, because clinically important disease is not confined to patients with documented risk factors. [502]
Diagnostic and etiologic surveillance
Conventional microbiology may underdiagnose pneumococcal pneumonia. A prospective adult CAP study evaluated quantitative PCR targeting the pneumococcal lytA and piaB genes in nasopharyngeal, oropharyngeal, saliva, and sputum specimens, using specimen-specific low-cycle-threshold cutoffs derived from control participants to reduce misclassification from asymptomatic carriage. [525] Molecular surveillance should therefore use validated, sample-specific thresholds and interpret positive results in the context of carriage controls and clinical presentation. [525]
Respiratory-virus surveillance is also relevant to CAP prevention planning. A systematic review and meta-analysis of human metapneumovirus (hMPV) infection included studies published from 2001 through June 2025 and assessed prevalence, case-fatality rate, age-related severity, and seasonality in acute respiratory infection and CAP populations. [515] Pediatric surveillance should account for post-pandemic changes in respiratory-pathogen transmission: a Changzhou cohort of hospitalized children with CAP compared pathogen patterns before and after COVID-19, while a Shanghai study evaluated adenovirus types, normalized viral load, and severe CAP outcomes in hospitalized children from 2021–2023. [459][492]
Bacterial surveillance should include geographically specific resistance monitoring. In Baoding, China, analysis of 13,534 children and adolescents aged ≤18 years with CAP from 2022–2024 found that Mycoplasma pneumoniae cases increased from 844 in 2022 to 6,346 in 2024, with positivity remaining between 67.68% and 77.72%; children aged 5–18 years had the highest positivity. [516] Genomic surveillance in China likewise examined the 2023 resurgence of macrolide-resistant M. pneumoniae using 685 isolates and historical and global sequences. [507] These findings support continued age-stratified molecular testing and resistance surveillance when atypical pneumonia is suspected. [516][507]
Legionella surveillance can be enhanced by molecular testing. A multicenter cohort evaluated a FRET-based real-time PCR assay targeting the Legionella 5S rRNA gene, using melting temperature analysis to distinguish species and assess associations with disease severity; among 51,489 PCR tests, 823 (1.6%) were positive. [501] Such testing may improve species-level epidemiologic surveillance, particularly when culture is negative or unavailable. [501]
| Group or pathogen | Evidence-informed surveillance focus |
|---|---|
| Adults ≥65 years | Compare enhanced influenza-vaccine strategies and assess pneumococcal-vaccine associations with severe CAP. [302]A1b[503] |
| CKD stages 1–5, not on dialysis | Assess serologic and clinical vaccine outcomes. [415]B2a |
| People with HIV | Stratify CAP incidence by age, CD4 count, and antiretroviral-therapy duration. [417]B2b |
| Adults with cancer | Monitor pneumococcal serotypes using urinary antigen assays. [416]B2b |
| Children and adolescents ≤18 years | Track M. pneumoniae positivity, age distribution, and macrolide resistance. [516][507] |
| Suspected pneumococcal or Legionella CAP | Consider validated qPCR approaches with carriage-aware thresholds or species-discriminating analysis. [525][501] |
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