Abstract
BACKGROUND: Respiratory infections, particularly those caused by influenza A virus (IAV), represent a major global health concern, contributing to substantial morbidity, mortality, and economic burden on healthcare systems worldwide. METHODS: We conducted a retrospective cohort study of 21,191 patient records in Chongqing, spanning a five-year surveillance period. Logistic and Poisson regression analyses were performed to characterize IAV prevalence across age-stratified, gender-specific, and clinically defined subgroups. In addition, restricted cubic spline (RCS) models were applied to examine the nonlinear effects of meteorological factors on IAV transmission. RESULTS: The overall IAV positivity rate was 13.79%, with marked differences across demographic subgroups. Preschool children (ages 4-6) showed the strongest association with IAV infection (OR = 6.25, 95% CI: 4.98-7.85; RR = 5.14, 95% CI: 4.13-6.41; both P < 0.001), followed by school-aged children (ages 7-18; OR = 5.60, 95% CI: 4.45-7.06; RR = 4.70, 95% CI: 3.76-5.88; both P < 0.001). Following the relaxation of COVID-19 restrictions, IAV positivity rebounded to 21.66% in 2023, with epidemic peaks observed in March-April and November-March. Four distinct epidemic waves were identified during the 2020-2024 surveillance period. RCS models revealed significant nonlinear associations between IAV prevalence and mean temperature, temperature variation, and mean relative humidity (all P for overall and nonlinear < 0.05). The highest positivity rates occurred at mean temperatures of 7.16-16.80 °C, temperature differences > 10.92 °C, and relative humidity levels of 61.91-74.00% (all P < 0.05). Furthermore, IAV infection was significantly more common in female patients compared with male patients (14.79% vs. 12.57%), and among patients with upper respiratory tract infections compared with those with lower respiratory tract infections (15.84% vs. 11.42%). CONCLUSIONS: These findings highlight the importance of targeted public health interventions and sustained surveillance to reduce the burden of IAV-related respiratory infections and improve patient outcomes.