Spatiotemporal trends in severe complicated influenza among the local population in Taiwan region, 2003-2023

OBJECTIVES: Severe influenza has raised considerable concern worldwide, and its incidence appears to have shifted in the context of globalization. This study aimed to examine the temporal, spatial, and demographic distributions of local severe influenza cases in Taiwan region from January 2003 to June 2023. METHODS: We aggregated severe complicated influenza cases by month, area (city/county), age, and sex. The age-standardized incidence rate (ASIR) was calculated to compare differences across regions and populations. Yearly incidence rate ratios comparing males to females were also computed to assess sex differences. RESULTS: A total of 16,459 cases were included from 2003 to 2023. Crude incidence rates per 100,000 population were 0.07-0.14 for 2003-2008, 3.64-9.81 for 2009-2019, and 0.00-1.87 for 2020-2023. Higher incidence rates were observed in Hualien and Taitung Counties, with average ASIRs exceeding 10.00 per 100,000 population, compared to other cities. Except for 2005 and 2007, the incidence among males exceeded that among females, with ASIR ratios ranging from 1.10 to 2.20. The highest incidence was observed among populations aged 0-4 and those aged ≥55. CONCLUSIONS: The incidence of severe complicated influenza exhibited clear regional and demographic variations in Taiwan region. The observed rebound in incidence calls for increased vigilance to protect vulnerable populations from severe illness.