Immunity Debt Regarding the Aspect of Influenza in the Post-COVID-19 Era in Taiwan

Immunity debt for various viral infections was reported globally in the post-COVID-19 era, but the data about influenza are lacking. This study collected data from Taiwan's CDC Open Data Portal. We analyzed the weekly number of influenza hospitalizations from January 2017 to May 2024. We divided the study period into four phases: the pre-COVID-19 without influenza epidemics, pre-COVID-19 with an influenza epidemic, COVID-19 pandemic lockdown control, and COVID-19 pandemic unlock periods. The Wilcoxon rank-sum test and interrupted time series analysis were used. The median case numbers of the four time periods were 174 (IQR = 98), 431 (IQR = 160), 8, and 155 (IQR = 175), respectively. Under the COVID-19 pandemic lockdown control, the weekly influenza hospitalization case number decreased by 90.2% (p < 0.001). The non-pharmaceutical intervention (NPI) policies during the COVID-19 pandemic helped Taiwan reduce influenza hospitalizations significantly. Till now, a comparison of the prevalence of influenza pre-COVID-19 and post-COVID-19 has yet to be reported. In our study, with the pandemic unlocking, it increased by 20-fold (p < 0.001), but the case number was still significantly lower than that pre-COVID-19. Amongst other factors, this may be associated with continuing self-induced NPIs in Taiwan.