The 2024–25 chikungunya epidemic on Réunion Island has become an unusually well-documented case study in how difficult it can be to turn early warning into timely public-health action during a fast-moving arboviral outbreak. In a Commentary published in Archives of Public Health, a team of French infectious-diseases and public-health researchers argues that the island’s experience, despite some of the most intensive outbreak surveillance anywhere in the region, exposes gaps that other territories increasingly exposed to mosquito-borne viruses cannot afford to ignore. The authors draw three central lessons from the epidemic: the challenge of translating surveillance signals into operational response, the pressure that surging case numbers placed on primary care, and the complexity of deploying a newly authorized vaccine in the middle of an ongoing epidemic.
The chronology of the outbreak is central to the analysis. The first autochthonous chikungunya cases, meaning locally acquired infections rather than imported ones, were detected on 23 August 2024. Local transmission then progressively intensified, and Réunion officially entered the epidemic phase on 13 January 2025. After several months of sustained transmission, declining epidemiological indicators led the prefect of Réunion, on the recommendation of the Regional Health Agency (ARS), to return the ORSEC arbovirus response plan to level 2 on 27 June 2025, formally marking the end of the epidemic phase. Even then, low-level viral circulation persisted on the island after the official phase had ended.
The burden of the epidemic was substantial. It resulted in 54,250 laboratory-confirmed cases, an estimated 195,800 chikungunya-related primary-care consultations, and nearly 3,000 emergency-department visits. Preliminary hospital and Santé publique France surveillance data, which the authors note are still being consolidated, indicate approximately 1,200 hospitalizations and 45 deaths. A post-epidemic serosurvey estimated overall chikungunya virus seroprevalence at 66.0 percent, of which approximately 46 percentage points were estimated to be attributable to the 2025 epidemic after accounting for residual immunity from the 2005–2006 epidemic that also struck the island. A population-weighted survey conducted at the end of the epidemic estimated a symptomatic attack rate of approximately 44 percent, with marked geographical heterogeneity across the island.
The serosurvey also addressed a question that often arises during epidemics: whether some age groups are disproportionately infected. Although recent infection varied across age groups in unadjusted analyses, age was not independently associated with infection after adjustment. In other words, once other factors were accounted for, being older or younger did not by itself determine the likelihood of infection. This distinction between crude, unadjusted patterns and adjusted analytical findings is one the authors take care to preserve, and it matters for anyone interpreting who was actually at greatest risk of acquiring the virus during this outbreak.
What made Réunion unusual was not only the size of the epidemic but the density of monitoring surrounding it. The outbreak was tracked through complementary systems combining laboratory-confirmed case reporting, primary-care and hospital surveillance, virological characterization of the circulating virus, vector surveillance, serological investigations and, once vaccination began, pharmacovigilance. Epidemiological and healthcare indicators were monitored in near real time during the epidemic, while population-level immunity was assessed afterwards through a dedicated post-epidemic serosurvey. This layered architecture allowed early detection of autochthonous transmission in August 2024, rapid characterization of the circulating chikungunya virus lineage, and close tracking of epidemic dynamics as they unfolded.
Yet the first lesson the authors draw is a sobering one about the limits of detection. Integrated epidemiological and virological surveillance worked as designed, identifying local transmission early and characterizing the virus quickly. However, the subsequent acceleration of transmission illustrates that interventions requiring substantial operational preparation, including vaccination, may be difficult to deploy rapidly once epidemic growth has begun. The authors are careful about what this does and does not show: rather than demonstrating a generalized delay in the response, the epidemic timeline highlights the importance of defining response strategies, decision thresholds and operational mechanisms before epidemic escalation, so that they can be activated rather than constructed while case numbers climb.
The second lesson concerns healthcare capacity. Around the epidemic peak, chikungunya-related consultations accounted for 18 percent of total general-practitioner activity, with an estimated 20,520 consultations for clinically compatible chikungunya cases in epidemiological week 15 alone. The authors note that the effect of this increased demand on access to care for other conditions was not quantitatively assessed, an important limitation, but they argue that these observations nonetheless demonstrate why primary-care surge capacity must be incorporated into arbovirus preparedness plans.
The third lesson is the most cautionary. The free IXCHIQ vaccination campaign began on 7 April 2025, initially targeting individuals at highest risk of severe disease, including adults aged 65 years and older, particularly those with comorbidities. Approximately 6,400 doses had been administered by late April. Previous acceptability data from Réunion had shown that willingness to receive chikungunya vaccination was strongly context-dependent, particularly on whether vaccination was provided free of charge, a finding the authors cite to underline the importance of affordability and delivery conditions when planning emergency vaccine deployment.
The safety trajectory of the campaign then unfolded quickly. Following reports of serious adverse events after immunization, French health authorities recommended on 26 April that IXCHIQ should no longer be administered to persons aged 65 years and older. Enhanced pharmacovigilance subsequently identified 47 reported adverse-event cases, including 18 serious cases and three deaths. Pharmacological assessment supported an association with IXCHIQ in 9 of the 18 serious cases, while an association was considered very likely for one of the three deaths. Similar safety concerns, particularly serious neurological and cardiac events in older adults, were also identified through post-authorization surveillance in the United States. For the authors, these events illustrate the importance of anticipating pharmacovigilance, benefit–risk reassessment and communication strategies before, not after, a newly authorized vaccine is deployed during an epidemic.
The Commentary is explicit that Réunion should not be treated as a direct epidemiological model for mainland Europe. The island combines established Aedes albopictus mosquito populations with tropical climatic conditions that can sustain transmission over a prolonged period, whereas chikungunya transmission in mainland Europe remains strongly seasonal and dependent on the introduction of viraemic cases into areas where competent vectors are active. Population immunity, geography and healthcare organization also differ substantially between settings. Nevertheless, the authors argue that the operational lessons travel well: translating surveillance signals into predefined response thresholds, anticipating healthcare surge capacity, preparing vaccine deployment and pharmacovigilance mechanisms in advance, ensuring rapid data sharing, and embedding research within outbreak response. These preparedness principles may be relevant beyond Réunion, but their implementation must be adapted to local epidemiological and health-system contexts.
The conclusions the authors reach are deliberately operational rather than dramatic. Effective preparedness, they write, requires more than early detection. Surveillance signals need to be linked to predefined escalation thresholds and operational response mechanisms; primary-care and hospital surge capacity should be anticipated; and vaccine-deployment strategies, including logistics, pharmacovigilance, benefit–risk reassessment and communication, should be established before epidemic acceleration. Timely data sharing across epidemiological, virological and healthcare surveillance systems is essential, while clinical and vaccine-effectiveness studies should be designed for rapid activation and embedded within the outbreak response itself. For regions increasingly exposed to chikungunya, the Réunion experience suggests that preparedness depends not only on detecting the next outbreak early, but on having the mechanisms in place to act on that information rapidly. As chikungunya transmission now occurs in new areas of Europe, the island stands as a sentinel at the crossroads of intense regional mobility, climate-driven vector expansion and increasing arboviral emergence.
Subject of Research: Lessons for arboviral outbreak preparedness from the monitored 2024–25 chikungunya epidemic in Réunion Island
Article Title: What a highly monitored chikungunya epidemic reveals about preparedness for Europe and beyond
Article References: Mosnier, E., Gérardin, P., D’Ortenzio, E., Pasquet, A., Deparis, X., & Yazdanpanah, Y. (2026). What a highly monitored chikungunya epidemic reveals about preparedness for Europe and beyond. Archives of Public Health, 84(1), Article 224. https://doi.org/10.1186/s13690-026-02119-3
Image Credits: AI Generated
DOI: 10.1186/s13690-026-02119-3
Keywords: chikungunya, Réunion Island, arbovirus, outbreak preparedness, surveillance, IXCHIQ vaccine, pharmacovigilance, primary care, Aedes albopictus, vector control, Europe, public health
