TRENDS-Thai: decadal trends of dengue, chikungunya, and hand, foot, and mouth disease in Thailand (2016-2025): a multi-disease time-series analysis of COVID-19 disruption

Introduction: Dengue, chikungunya, and hand, foot, and mouth disease (HFMD) are priority notifiable infections in Thailand. Whether vector-borne and contact-mediated diseases responded differently to the coronavirus disease 2019 pandemic has not been quantified within a unified national surveillance framework over an extended period. Methods: We conducted an ecological interrupted time-series analysis using weekly province-level notifiable disease surveillance data from epidemiological week 1 of 2016 to week 53 of 2025 across all 77 Thai provinces. Incidence per 100,000 population was calculated using year-specific civil registration population denominators. Segmented quasi-Poisson regression with two Fourier harmonics for annual seasonality was fitted, with the primary pandemic onset defined as week 1 of 2020 and two alternative onset definitions prespecified for sensitivity analysis. Results: The analysis included 40,579 province-week observations across 527 epidemiological weeks, comprising 790,263 dengue, 32,265 chikungunya, and 713,822 HFMD cases nationally. Immediate incidence rate ratios at pandemic onset were 0.39, 0.54, and 0.51 for dengue, chikungunya, and HFMD, respectively. Sustained post-onset trends diverged across diseases, with declining trajectories for the two vector-borne infections and a positive post-onset slope for hand, foot, and mouth disease. Dengue rebounded above pre-pandemic levels by 2023, chikungunya remained quiescent through 2025, and HFMD exceeded its pre-pandemic baseline by approximately 26%. Conclusion: Vector-borne and contact-mediated diseases in Thailand followed sharply contrasting decadal trajectories that tracked the transmission ecologies of each pathogen. These findings support transmission-mode-specific pandemic-resilient surveillance, accelerated arboviral and enteroviral vaccine deployment, and integrated vector management.