Between August 2024 and June 2025, Réunion Island recorded more than 54,000 confirmed chikungunya cases and 43 deaths during a massive mosquito-borne epidemic, according to data from the Université Libre de Bruxelles and the Arbovirus National Reference Center. The outbreak ranks as the second major event of its kind on the island following the 2005–2006 epidemic, which affected roughly a third of the local population.
Genomic Tracking and Viral Spread Dynamics
Researchers sequenced more than 3,000 near-complete viral genomes across Réunion Island during the outbreak, achieving a sampling fraction typically reserved for pathogens like Ebola and SARS-CoV-2. According to findings published in the Proceedings of the National Academy of Sciences by E. Frumence and colleagues, this genomic dataset allowed Belgian, French, and American researchers to apply phylodynamic and phylogeographic methods to trace the outbreak.
The analysis shows that the epidemic likely originated from a single introduction of the virus. From there, the pathogen circulated through frequent movements between municipalities, driven largely by human mobility and high population density. Researchers noted that this human-driven spread became especially pronounced at the peak of the epidemic, with repeated viral exchanges occurring between distant residential areas. By 2025, this same Réunion epidemic lineage spread to other Indian Ocean islands and later fueled unprecedented case numbers in more temperate regions of Europe and in China over the summer months.
Climate Factors and Population Immunity
Scientists investigated why the outbreak eventually receded, evaluating both environmental conditions and immunity levels. Climate models indicate that temperature and precipitation—which dictate mosquito density and activity—shaped the epidemic’s dynamics, but researchers confirmed that climate alone cannot account for the drop in transmission. Instead, the gradual build-up of population immunity proved sufficient on its own to push the effective reproduction number below 1.
Did you know? An estimated 66% of Réunion Island’s population is now immune to the chikungunya virus. Researchers concluded that this high level of immunity makes the risk of a large resurgence low, a prediction supported by the following season, which saw only a handful of local cases despite ongoing virus circulation elsewhere in the Indian Ocean.
The study highlights the practical value of integrating genomic, demographic, environmental, and immunity data into standard epidemiological assessments. Authors point to continuous seroprevalence monitoring and targeted vector control as concrete tools for anticipating future shifts in transmission.
Frequently Asked Questions
What caused the chikungunya outbreak on Réunion Island?
The epidemic stemmed from a single introduction of the chikungunya virus, which is transmitted by Aedes mosquitoes. Human mobility and high population density drove the rapid spread of the virus between municipalities across the island.
How many people were affected by the 2024–2025 epidemic?
The outbreak resulted in more than 54,000 confirmed cases and 43 deaths between August 2024 and June 2025.
Is there a high risk of another major outbreak on the island soon?
Researchers concluded that the risk of a large resurgence is low because an estimated 66% of the population has developed immunity to the virus.
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