Chikungunya likely entered Réunion Island once then spread through human movement, study suggests

by Université libre de Bruxelles

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Between August 2024 and June 2025, Réunion Island experienced a massive chikungunya epidemic, with more than 54,000 confirmed cases and 43 deaths. It was the island's second major chikungunya virus outbreak, after the 2005–2006 epidemic, which affected about a third of the island's population.

Chikungunya is a mosquito-borne virus transmitted by Aedes mosquitoes. While rarely fatal, it causes severe joint pain that can persist for months or years. In 2025, the Réunion epidemic lineage reached other Indian Ocean islands and, over the summer, fueled unprecedented numbers of cases in more temperate regions of Europe and in China.

Throughout the outbreak, the associated Arbovirus National Reference Center in Réunion carried out intensive genomic surveillance, sequencing more than 3,000 near-complete viral genomes from across the island. The share of confirmed cases sequenced was comparable to that for only a handful of other pathogens, such as Ebola and SARS-CoV-2. This dataset provided a strong basis for tracking how the virus evolved, spread and eventually receded.

To make use of these data, the team of Belgian, French and American researchers applied phylodynamic and phylogeographic approaches (statistical methods that use the evolutionary relationships between viral genomes to reconstruct where and how a pathogen has traveled). The analyses indicate that the epidemic likely stemmed from a single introduction and that the virus then circulated in a highly intermixed fashion, with frequent movement between municipalities driven by human mobility and population density. This human-driven spread was especially visible at the peak of the epidemic, with repeated viral exchanges between distant residential areas.

The study is published in the journal Proceedings of the National Academy of Sciences.

The researchers also examined why the epidemic ended. Their models show that the epidemic's dynamics were associated with climatic conditions (temperature and precipitation, which shape mosquito density and activity), but climate alone cannot account for the decline in transmission. In theory, the gradual buildup of population immunity was sufficient on its own to push the effective reproduction number below 1, bringing the outbreak to an end.

With an estimated 66% of the population now immune to the virus, the authors conclude that the risk of a large resurgence is low. This prediction was borne out by the following season, when only a handful of local cases were reported despite continued chikungunya virus circulation elsewhere in the Indian Ocean.

The study shows the value of integrating genomic, demographic, environmental and immunity data into epidemiological assessments. It also points to tools for the future, such as continuous seroprevalence monitoring to anticipate shifts in transmission and targeted mosquito control.

Publication details

Etienne Frumence et al, Unraveling the epidemiological and dispersal dynamics of the 2024–2025 chikungunya virus epidemic on Réunion Island, Proceedings of the National Academy of Sciences (2026). DOI: 10.1073/pnas.2621019123

Journal information: Proceedings of the National Academy of Sciences

Key medical concepts

Chikungunya virusImmunity, Herd

Clinical categories

Infectious diseasesCommon illnesses & PreventionPreventive medicine Provided by Université libre de Bruxelles Who's behind this story?

Gaby Clark

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Citation: Chikungunya likely entered Réunion Island once then spread through human movement, study suggests (2026, September 25) retrieved 25 September 2026 from https://medicalxpress.com/news/2026-09-chikungunya-runion-island-human-movement.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.