Contrasting pandemic-era trajectories of dengue, chikungunya, and hand, foot, and mouth disease in Thailand using national surveillance findings, 2016–2025

Objectives To compare the pandemic-era trajectories of dengue, chikungunya, and hand, foot, and mouth disease in Thailand Methods We conducted a national ecological interrupted time-series analysis using weekly surveillance data from all 77 Thai provinces from 2016 to 2025. Segmented quasi-Poisson regressi​on with Fourier terms for annual seasonality estimated level and slope changes at a calendar-based 2020 breakpoint; sensitivity analyses used later breakpoints. Results The analysis included 40,579 province-week observations including 790,263 dengue, 32,265 chikungunya, and 713,822 hand, foot, and mouth disease cases. The estimated level incidence rate ratios at week 1 of 2020 breakpoint were 0.39 for dengue (95% confidence interval 0.32–0.48), 0.54 for chikungunya (95% confidence interval 0.41–0.71), and 0.51 for hand, foot, and mouth disease (95% confidence interval 0.38–0.69). Dengue rebounded above pre-pandemic levels by 2023, chikungunya declined after the 2018–2020 outbreak, and mean annual incidence of hand, foot, and mouth disease in 2023–2025 exceeded the pre-pandemic baseline by approximately 26%. Conclusions These contrasting trajectories support the need for comparative multi-disease surveillance. Ecological designs cannot separate the contribution of transmission ecology from epidemic history or other time-varying factors.