Increased Mortality Rate During 2025 Chikungunya Epidemic in Réunion Island

Background: Chikungunya virus (CHIKV) has historically been regarded as a low-fatality infection; however, growing evidence from diverse study designs demonstrates a substantial mortality burden during large-scale epidemics. In 2025, Réunion Island experienced a major CHIKV outbreak, raising renewed concerns about its fatal impact. Methods: We conducted an ecological time-series analysis of all-cause mortality in Réunion Island from March to May 2025, focusing on individuals aged ≥65 years. Excess deaths were estimated using a log-linear model adjusted for long-term trends and seasonality, with a baseline derived from the preceding five years. Observed deaths were compared to officially reported CHIKV fatalities and contextualized with data from previous epidemics worldwide. Results: An estimated 208 excess deaths occurred among older adults during the three-month epidemic, corresponding to a mortality rate ratio of 1.34 (95% CI: 1.22–1.47; p < 0.000001) in April 2025. Officially, only 27 deaths were attributed to CHIKV during this period (17 direct, 10 indirect), highlighting a substantial discrepancy between excess and reported mortality. Similar patterns have been repeatedly documented across different regions, including India (2006), the French Caribbean (2014), Puerto Rico (2014), Brazil (2015–2023), Jamaica (2014), and Mauritius (2006), where thousands of excess deaths far exceeded confirmed counts. Conclusions: Chikungunya epidemics are consistently associated with substantial underrecognized mortality worldwide. Routine surveillance relying solely on laboratory confirmation underestimates the true burden of disease. Integrating excess mortality analysis, strengthening diagnostic and postmortem investigations, and implementing timely mitigation measures are essential to accurately assess and reduce preventable deaths during future CHIKV outbreaks.