Shifting seasonal patterns of influenza before, during, and after the COVID-19 pandemic in Shanghai, China: A population-based surveillance study, 2015–2023

Seasonal influenza activity was disrupted due to the emergence of coronavirus disease 2019 (COVID-19) and the implementation of non-pharmaceutical interventions. In this study, we analyzed weekly influenza-like illness (ILI) cases and influenza positivity rate by subtype and age group from the Shanghai influenza surveillance system from 2015 to 2023. A wavelet analysis was conducted to explore the seasonal pattern of influenza. The average weekly ILI consultation rate was 20.28 ILI cases per 1,000 consultations, with 40,778 of 173,155 specimens (22.63 %) testing positive for influenza. During the COVID-19 pandemic, nearly no influenza viruses were detected until January 2021. Only B/Victoria was circulated for about eight months and peaked in January 2022, with a positivity rate of 47.48 %, which was lower than the peaks in each year before the COVID-19 pandemic. In 2023, the first epidemic, dominated by A(H1N1), began in week 8 (one month later than the 8-year average) and peaked at 76.89 %. The second epidemic, which was unusually not summer-based and driven by A(H3N2), started in week 38 and peaked in week 48 at 75.31 %, advancing the winter-spring epidemic by approximately one month. The age preference of influenza A changed during the era of normalized COVID-19 transmission. The seasonal patterns of influenza varied significantly across different stages of the COVID-19 pandemic, including differences in epidemic onset, peak timing, duration, periodicity, and dominant strains. Stability of seasonal changes in influenza requires longer-term surveillance, as well as further virologic and epidemiologic studies.