Pneumococcal pneumonia trends in adults hospitalised with community-acquired pneumonia over 10 years (2013-2023) and the role of serotype 3

Background: With higher valency pneumococcal vaccines on the horizon and new adult immunisation strategies under discussion, we aimed to evaluate the contribution of individual pneumococcal serotypes to the burden of pneumococcal community-acquired pneumonia (CAP). Over 10 years, trends in pneumococcal pneumonia epidemiology in adults hospitalised with CAP were assessed. The risk factors and severity associated with serotype 3 were examined.

Methods: We conducted a prospective cohort study of adults hospitalised with CAP between September 2013 and May 2023. Pneumococcal serotypes were identified using a serotype-specific 24-valent urinary-antigen assay. Trends in the proportion of CAP due to pneumococcus and causative serotypes were compared prepandemic and postpandemic. Risk factors and severity of serotype 3 pneumonia were compared with other serotypes using logistic regression.

Results: Of 5186 patients with CAP, 2193 (42.2%) had pneumococcal pneumonia. The proportion of CAP due to pneumococcus increased across all ages between 2013 and 2023 (36.4%-66.9%, p<0.001). The proportion due to serotype 3 increased significantly from 13.4% (2013) to 48.8% (2023). Serotype 3 pneumonia in adults was associated with older age (p<0.001), male sex (adjusted OR (aOR) 2.22, 95% CI 1.64 to 3.01) and chronic renal disease (aOR 1.81, 95% CI 1.09 to 3.02). Serotype 3 pneumonia was not observed to be associated with severity, critical care requirement, mortality or readmission.

Interpretation: Serotype 3 is the predominant serotype in adult pneumococcal CAP and has been increasing despite a mature infant pneumococcal immunisation programme, consistent with a lack of herd protection for this serotype.