Global and regional burden of chikungunya from 2004 to 2024: a worldwide observational study.

Background: Chikungunya has emerged as a growing global health threat with a new sharp rise in outbreaks across 119 countries. However, its transmission patterns remain poorly characterised. We aimed to describe the global burden and spatiotemporal trends of chikungunya, and identify country-level environmental and socioeconomic factors associated with local transmission. Methods: We compiled annual country-level autochthonous chikungunya cases from 2004 to 2024, using data from regional surveillance systems and peer-reviewed sources. We calculated the incidence rates using the number of new cases and the population. We employed a generalised additive model (GAM) to flexibly model nonlinear associations between chikungunya incidence and environmental and socioeconomic factors. We performed subgroup analyses across the six WHO regions and conducted multiple sensitivity analyses addressing data structure, variable selection, and alternative model specifications to assess robustness. Results: Between 2004 and 2024, the global incidence of chikungunya from autochthonous cases rose from 0.28 to 11.13 per 100 000. In 2024, the Americas bore the heaviest burden (43.9 per 100 000; 431 305 cases), followed by South-East Asia (14.3 per 100 000; 258 854 cases), while Africa, Europe, and the Western Pacific reported few cases. Local outbreaks occurred recurrently in several countries across the Americas, South-East Asia, and Western Pacific, but remained sporadic in temperate regions. Using a GAM, we identified significant nonlinear effects of temperature, urbanisation, and GDP per capita on incidence: incidence rose sharply above 17°C; urban population percentage demonstrated a complex, nonlinear relationship; and GDP showed an inverse association at low to moderate levels. After adjusting for environmental and socioeconomic factors, the temporal trend of incidence generally declined. We identified notable heterogeneity across regions, while our results otherwise remained broadly consistent across the sensitivity analyses. Conclusions: Chikungunya burden has expanded globally, shaped by environmental and socioeconomic factors. Strengthened surveillance, integration of climate information into preparedness efforts, and improvements in socioeconomic conditions are needed to reduce disparities and support more effective prevention of future outbreaks.