Characteristics of respiratory co-infections in influenza-like illness during the post-COVID-19 era

BACKGROUND: In the postpandemic era, populations may experience an "immunity debt" due to reduced social interactions. Studying respiratory co-infections in populations is of great significance for precise treatment and the prevention and control of infectious disease outbreaks. METHODS: A prospective surveillance network was established across 24 sentinel hospitals. Respiratory specimens were collected. Nucleic acids were extracted using magnetic bead-based methods and tested by real-time PCR for 10 viruses and 5 bacterial targets. Data were analyzed to determine positivity rates, co-infections, and age-specific patterns using Join-Point Regression. RESULTS: The overall pathogen positivity rate was 64.3% (9954/15,481). Co-infection with ≥2 pathogens was detected in 26.7% of patients. Influenza virus (IFV) was the predominant virus (20.9% overall), peaking in January 2025 (49.4%). Human metapneumovirus was the second most common virus (8.0%). Haemophilus influenzae (21.1%) and Streptococcus pneumoniae (17.7%) were the most frequently detected bacteria. Positivity rates exhibited significant age-related variation. Join-Point analysis revealed that the IFV positivity rate increased until age 50. Considerable geographic variation was observed, with the total positivity rate ranging from 53.2% to 82.2% across different cities. CONCLUSION: This comprehensive surveillance revealed a high burden of respiratory pathogens and complex co-infection patterns in the postpandemic era.