Unravelling the epidemiological and dispersal dynamics of the 2024-2025 chikungunya virus outbreak on Reunion island

Reunion island just experienced a massive chikungunya virus outbreak in 2024-2025, with more than 54,000 confirmed cases. This is the second major chikungunya outbreak on the island, following the first one that peaked 20 years ago. It has been assessed that this new outbreak finds its origin in a single introduction event into the island, offering a unique opportunity to exploit viral genomic data to understand the epidemiological and dispersal dynamics of the introduced transmission chain. We sequenced >3,000 near-full viral genomes collected during the course of the epidemic. Harnessing this genomic dataset, we used a set of phylodynamic and phylogeographic approaches to unravel the paths taken by the transmission chain and the external factors having impacted its dynamics on the island. Our analyses highlight a dispersal pattern in line with a gravity-model dynamic with viral transition events being more frequent from and toward more populated areas. While we find that dispersal events were on average more likely between geographically close locations, our analyses also reveal that the transmission chain was overall spatially intermixed, with frequent exchanges among distant residential areas. In addition, we show that the decrease in transmission rate leading to the end of the epidemic can, at least to a large extent, be attributed to the population immunity resulting from both the current and the 2005-2006 epidemic. While a short-term resurgence of viral transmission cannot be excluded, the impact of herd immunity constitutes an encouraging outcome that should at least contribute to limiting the spread of the virus in the upcoming seasons.