Resurgence of Influenza Following the Lifting of Non-pharmaceutical Interventions in Shenzhen, China

BACKGROUND: During the coronavirus disease 2019 (COVID-19) pandemic, global influenza activity sharply declined due to extensive non-pharmaceutical interventions (NPIs). Understanding how influenza activity rebounded after these interventions were lifted is critical for informing future respiratory virus control strategies. METHODS: We conducted a descriptive analysis of the temporal characteristics of weekly number of influenza-like illness (ILI) cases, ILI%, influenza-positive cases, and influenza-positive rates. Poisson log-link regression models, incorporating meteorological factors using data from 2013 to 2019, were established to predict the weekly influenza-positive rate under a counterfactual scenario without COVID-19 interventions in 2022-2023. RESULTS: Our findings indicate that the cancellation of COVID-19-related NPIs had a notable impact on increasing influenza transmission. Children under 5 years old exhibited the highest ILI cases. The influenza positivity rate surged to 34.35% during the pandemic relaxation, surpassing pre-pandemic (24.53%) and pandemic (9.56%) rates. During the pre-COVID-19 period, various influenza virus subtypes were co-circulated, with the predominant subtype varying. However, during the COVID-19 pandemic period, the dominant strains were influenza A/H1N1 and influenza B/Victoria lineage, while influenza A/H3N2 predominated in the pandemic relaxation period. CONCLUSION: The marked resurgence of influenza activity in Shenzhen following the lifting of COVID-19-related NPIs underscores the need for sustained surveillance and preparedness for concurrent or sequential respiratory virus outbreaks.