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A massive chikungunya epidemic swept Réunion Island between August 2024 and June 2025, infecting over 54,000 people and killing 43. Researchers sequenced more than 3,000 viral genomes to track how the virus spread and eventually burned out. Their findings suggest that population immunity — not just climate — was the key force that ended the outbreak.
Réunion Island endured its worst chikungunya outbreak in nearly two decades, with over 54,000 confirmed cases and 43 deaths recorded between August 2024 and June 2025. The virus — spread by Aedes mosquitoes and notorious for causing debilitating joint pain lasting months or years — later rippled outward to other Indian Ocean islands, parts of Europe, and China.
To understand how the epidemic unfolded, a team of Belgian, French, and American researchers conducted one of the most intensive genomic surveillance efforts ever mounted for an arbovirus, sequencing more than 3,000 near-complete viral genomes. Their analysis revealed that the outbreak likely stemmed from a single viral introduction, with the virus then spreading rapidly across municipalities, driven by human mobility and population density rather than geography alone.
Crucially, the study found that while climate factors like temperature and rainfall shaped mosquito activity, they alone couldn't explain the epidemic's end. It was the gradual buildup of population immunity that ultimately pushed transmission below the threshold needed to sustain the outbreak.
By the Numbers:
Why it matters: This study is a blueprint for future outbreak response — showing how combining genomic, demographic, environmental, and immunity data can not only explain how an epidemic spreads, but also predict when and why it ends.
