Epidemiological shift in age distribution of Dengue in Thailand, 2005-2024.

Dengue has historically been characterized as a childhood disease, but evidence of shifting age distribution is increasing. We analyzed 20 years (2005-2024) of national dengue surveillance data from Thailand comprising 1,639,442 cases to examine temporal trends in age-specific incidence, quantify age shift magnitude using the ratio of incidence rate ratios (RIRR), and decompose observed changes using Kitagawa decomposition. The proportion of cases among adults ≥25 years nearly doubled from 19.56% to 36.16%, while the children's proportion declined from 54.49% to 41.63%. RIRR for adults 25-44 years was 1.78 (95% CI 1.76-1.80), and for older adults ≥45 years was 2.19 (95% CI 2.15-2.24). Kitagawa decomposition attributed +178.7% of change to epidemiological factors and -78.7% to demographic shifts, supporting a declining force of infection rather than population aging as the primary driver. These findings necessitate expanding dengue prevention beyond school-based programs and enhancing clinical vigilance for adult dengue.