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

Abstract

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.

Keywords: Epidemiology; Incidence; Severe influenza.

Figure 1.

Annual number and incidence of severe complicated influenza cases in 2003-2023. For 2023, only cases with onset from January to June are included. Bars represent the number of cases, while the line depicts the crude incidence rate. The study period is divided into 3 segments: pre-2009 H1N1 pandemic (2003-2008), during and post-2009 H1N1 pandemic (2009-2019), and the coronavirus disease 2019 (COVID-19) pandemic (2020-2023).

Figure 2.

Matrix of monthly severe complicated influenza cases (2003-2023). Each cell shows monthly case counts. Red and blue grids indicate hotspots and cold spots, respectively; the deeper the red (or blue), the higher (or lower) the number of cases. Due to disparities in case counts throughout the study period, the matrix is divided into 3 separate periods. The hotspot and cold spot designations apply only within their corresponding period.

Figure 3.

Spatial distribution of severe complicated influenza by period. For 2003-2008 and 2009-2019, the average annual age-standardized incidence rates (ASIR) are displayed. For 2020-2022 and 2023, both the annual ASIR and the cumulative incidence rate (from January to June) are shown.

Figure 4.

Age-standardized incidence rate ratio (male to female) of severe complicated influenza in 2003-2023. The red dashed line indicates a ratio of 1.00. No value is shown for 2021 because the numerator was 0.

Figure 5.

Crude incidence rate of severe complicated influenza and its association with age groups. Each set of bars represents the crude incidence rates for a specific age group, with individual bars showing the rates for each year from 2003 to 2023. A generalized additive model was used to examine the relationship between age groups (using the 25-29-year age group as the reference) and crude incidence rates, adjusting for year. The curve and shaded area represent the relative risk (RR) values and their 95% confidence intervals as they change with age.