Community-acquired pneumonia in Europe: heterogeneity, evidence gaps, and priorities for care

Community-acquired pneumonia (CAP) remains a leading cause of hospital and intensive care admission and infectious death in Europe, particularly among older adults, people with multimorbidity, and immunocompromised patients. This Review argues that persistent uncertainty in CAP evidence largely reflects heterogeneity across eight domains: case definition, host, pathogen ascertainment, severity, inflammatory response, treatment exposure, timing and trajectory, and outcome measurement. We examine how these domains distort incidence estimates, pathogen rankings, diagnostic accuracy, severity classification, and trial findings. Within this framework, we highlight developments relevant to Europe: randomised vaccine trials assessing hospitalisation rather than immunogenicity; national data suggesting a changing, increasingly β-lactam-resistant role for Haemophilus influenzae ; evidence suggesting that discordant corticosteroid trials reflect heterogeneity of treatment effect rather than drug or protocol differences; and the first randomised trial of host-response-directed immunotherapy in pneumonia-predominant sepsis. We also examine continuing fragmentation in European adult vaccination policy, surveillance, and diagnostic capacity. We propose a diagnostic approach organised by clinical group, a minimum CAP dataset with explicit governance, and a research agenda separating interventions applicable now, evidence required before implementation, and unresolved questions. This approach connects advances in prevention, diagnosis, and treatment with the methodological and organisational changes needed to generate more reliable, clinically actionable European evidence.