Surge in Chlamydia pneumoniae infections in China: insights from 2022 to 2024.

Chlamydia pneumoniae (C. pneumoniae) remains an underrecognized cause of respiratory infections in China. As C. pneumoniae-associated pneumonia has not been classified as a notifiable infectious disease, nationwide surveillance data are unavailable, resulting in poorly characterized epidemiological patterns. Using targeted next-generation sequencing (tNGS) data from 2,316,182 hospitalized acute respiratory infection (ARI) cases across 315 cities in 31 provinces of mainland China, we characterized the seasonal epidemiology and demographic patterns of C. pneumoniae infections. Our analyses revealed a 4.3-fold increase in C. pneumoniae positivity rates in 2024; from 0.21% in 2022-2023 to 0.90% in 2024, with distinct geographic clustering in the southwestern regions of the Yangtze River Basin. Seasonal analysis identified the period from April to June as the peak season for C. pneumoniae infections. Demographic stratifications showed male adolescents aged 10-19 years emerged as the highest-risk population, while women aged 25-44 years also showed elevated infection risks likely due to caregiving roles. Our findings demonstrate the changes in the epidemiology of C. pneumoniae in post-pandemic China and underscore the necessity of enhancing surveillance of C. pneumoniae infections and implementing targeted prevention strategies, particularly in high-burden areas and high-risk populations such as adolescents/caregivers in southwestern regions of the Yangtze River Basin.