Age-Specific Patterns of Infection, Co-Infection and Pneumonia Risk in Acute Respiratory Infections: An Active Surveillance Study in China, 2023-2024.

Acute respiratory infections (ARIs) remain a major contributor to global morbidity and mortality, with diverse viral and bacterial pathogens varying across age groups. We conducted a multicenter, active surveillance study in Zhejiang Province, China, from November 2023 to November 2024, enrolling 31 490 patients with ARIs, including 6354 pneumonia cases. Specimens were tested for 13 respiratory pathogens, including 10 viruses and 3 bacteria, by polymerase chain reaction. The detection rate was calculated. Generalized additive models were used to explore the nonlinear relationship between age and detection rate, and modified Poisson regression was performed to evaluate pathogen infection and co-infection patterns associations with pneumonia risk. Overall, 57.49% of patients tested positive for at least one pathogen. Influenza virus (16.35%), Streptococcus pneumoniae (11.83%), and SARS-CoV-2 (9.04%) were the most frequently detected pathogens. Among pneumonia patients, Mycoplasma pneumoniae (M. pneumoniae) was predominant (24.80%), especially in adolescents (54.24%). Several pathogens exhibited distinct age-specific detection trends, with pediatric peaks for most viruses, adolescent predominance for IFV and M. pneumoniae, and higher detection of SARS-CoV-2 in older adults. Certain pathogens, such as RSV and M. pneumoniae, were positively associated with pneumonia. Co-infections were identified in 12.58% of patients, with the highest rate in children (22.80%), and viral-bacterial co-infections being the most common patterns (8.94%). Bacterial-related co-infection patterns were positively associated with pneumonia. These findings reveal distinct age-specific patterns of pathogen detection and co-infection in ARIs and support the need for age-targeted prevention strategies, including vaccination, optimized antibiotic use, and integrated surveillance approaches in the postpandemic era.