The effect of non-pharmaceutical interventions on influenza throughout the COVID-19 pandemic: an 8-year interrupted time series study

Abstract Limited information is available on the impact of non-pharmaceutical interventions (NPIs) and immune debt on influenza during COVID-19. This retrospective population-based research examined the effect of the COVID-19 pandemic on the prevalence of influenza-like illness (ILI) cases and influenza positive cases in Sichuan Province. Interrupted time series analysis (ITS) was conducted using data gathered from surveillance hospitals throughout 21 cities, including information on patients diagnosed with ILI and positive nucleic acid testing. Subsequent to the implementation of NPIs, there was a notable temporary reduction in both the incidence of ILI cases and confirmed influenza cases, decreasing by 94.7% and 98.0%, respectively (P<0.001). The weekly trend in influenza-positive cases was notable (+4.1%/week, P=0.001). Following the execution of the second-phase intervention, the fluctuations in the transient changes of ILI patients and influenza-positive cases varied; nonetheless, both subsequently exhibited a significant declining trend (P<0.001). Temperature had a substantial negative correlation with both indicators (P<0.001), resulting in a reduction of instances by 13.0% and 11.5%, respectively; humidity demonstrated no statistically significant impact on either. Our data demonstrate that NPIs significantly reduce two influenza outcome indicators in the short term; nevertheless, they do not effectuate a sustainable alteration in the long-term increasing trend of influenza. Through the examination of the dynamic effects of NPIs on influenza transmission, we provide policymakers with an enhanced evaluative instrument, a decision-making framework, and theoretical backing, while also contemplating the prospective advantages and hazards of NPIs for additional respiratory infectious diseases in the future.