Disease Outbreak News

Chikungunya - La Réunion and Mayotte

12 May 2025

Situation at a glance

Since August 2024, widespread transmission of chikungunya virus disease has been documented in La Réunion as well as increasing locally transmitted cases in Mayotte. Although chikungunya outbreaks and endemic transmission occur annually in several countries and territories around the world, the Indian Ocean islands have not experienced major outbreaks for nearly two decades. In La Réunion, over 47 500 cases and twelve associated deaths have been reported as of 4 May 2025, with sustained high transmission across the island. In Mayotte, the first locally transmitted cases since 2005–2006 have been detected, raising concern about similar large outbreaks. Public health response measures, including enhanced surveillance, vector control activities, and novel targeted vaccination efforts, have been implemented to contain the outbreaks, however further outbreak activity in the islands of the Indian Ocean can be expected.

Description of the situation

La Réunion, France

In August 2024, the regional health authority (Agence régionale de santé, ARS) of La Réunion, France, reported three locally transmitted chikungunya cases. Cumulatively, from the start of the outbreak in August 2024 through 4 May 2025, over 47 500 confirmed cases of chikungunya and more than 170 000 consultations for suspected chikungunya have been reported on the island. The surge in cases was observed from early 2025 with weekly case counts increasing from 30 at the end of 2024 to 4000 during the week of 10-16 March 2025 (epidemiological week 11), indicating more than a 100-fold increase.

The overall clinical presentation of the illness was generally mild; however, from 1 January to 4 May, 340 hospitalizations have been reported. Almost half (43%) of hospitalized patients were adults over 65 years old and approximately 25% were children under six months old. Over 95% of hospitalized patients had at least one risk factor for severe disease including comorbidity, age-group (over 65 years and infants) or pregnancy. Additionally, 74 pregnant women and 48 infants under six months were hospitalized for short-term monitoring (2-3 days).

To date, 66 severe cases (defined as those with at least one organ failure) have been reported. These included 36 adults over 65 years of age with comorbidities, 7 people under 65 years of age with comorbidities and 23 infants under 3 months of age.  Over the course of the outbreak, 12 deaths that occurred between 9 March and 27 April (Epi weeks 11-17) in people over 70 years of age with comorbidities were classified as linked to chikungunya, and additional twenty-eight deaths are currently under investigation for chikungunya, including one neonatal death.

The cumulative number of cases continues to increase, with the southern municipalities being the most affected by the virus, particularly Le Tampon. To date, all the municipalities have reported cases.

The previous large outbreak in La Reunion occurred in 2005-2006 with estimated total cases ranging between from 244 000 to over 300 000. 

In March 2025, imported cases as a result of travelers who had visited La Réunion were reported in Mayotte (2), Wallis and Futuna (1) and Martinique (1). 

Figure 1: Epidemiological curve of the number of confirmed chikungunya cases in La Reunion between 1 January and 27 April 2025 by week of symptom onset Epi curve Chikungunya La Réunion

Mayotte, France

As of 4 May 2025, a total of 116 chikungunya cases have been reported by ARS Mayotte, France, including 29 imported cases, 57 locally acquired cases, and 30 cases under investigation. The first imported case was identified on 5 March in the Northeast of the Island, followed by further cases reported across multiple communes. This marks the first locally transmitted chikungunya cases since the 2005–2006 outbreak, which had resulted in approximately 7300 cases. One hospitalization has been reported, with no deaths to date.

Epidemiology

Chikungunya is a mosquito-borne viral disease that causes fever and severe joint pain. It is caused by a ribonucleic acid (RNA) virus that belongs to the alphavirus genus of the family Togaviridae. Chikungunya virus (CHIKV) is transmitted to humans via the bites of infected female mosquitoes, most commonly Aedes aegypti and Aedes albopictus mosquitoes. These two species can also transmit other viruses, including dengue and Zika viruses. They bite primarily during daylight hours, and there may be peaks of activity in the early morning and late afternoon.

Symptoms of chikungunya appear between 4 and 8 days (range 2–12 days) after the bite of the infected mosquito. Most symptoms are generally self-limiting and last for 2-7 days. The disease is characterized by an abrupt onset of fever, which is frequently accompanied by severe joint pain. Other common signs and symptoms include stiffness, arthritis, headache, fatigue and rash. The joint pain is often debilitating and usually lasts for a few days but may be prolonged, lasting for weeks, months or even years. Because symptoms in infected individuals are similar to those seen in other arbovirus infections, chikungunya may be misdiagnosed in areas where dengue and other arboviruses also occur.

Although most patients recover fully from the infection, occasional cases of eye, heart and neurological complications have been reported with CHIKV infections. Patients at extremes of the age spectrum are at higher risk for severe disease, including newborns infected during delivery to infected mothers or bitten by infected mosquitoes in the weeks after birth, and older people with underlying medical conditions. Patients with severe disease require hospitalization because of the risk of organ damage that can be fatal.

Once an individual has recovered, available evidence suggests they are likely to be immune from future chikungunya infections. Treatment is directed primarily at relieving the symptoms, including analgesics for joint pain and anti-pyretic drugs to reduce fever. Aspirin and other non-steroidal anti-inflammatory drugs should not be administered until dengue can be ruled out to reduce the risk of bleeding. There is no specific antiviral drug treatment for chikungunya.

Public health response

La Réunion, France

On 13 January 2025, the prefecture of La Réunion activated level 3 of the Organisation de la Réponse de Sécurité Civile (ORSEC) arboviruses plan, indicating a low-intensity epidemic. This escalated to Level 4 on 14 March, signifying a medium-intensity epidemic. The ORSEC arboviruses plan is a specific subset of the departmental ORSEC plan, which is a comprehensive emergency response framework used in France. It is designed to combat mosquito-borne diseases such as dengue, chikungunya, and Zika. Its objectives include ensuring prevention and surveillance of arboviruses, coordinating stakeholders, and defining intervention methods. Since activating Level 4 of the ORSEC arboviruses plan, the regional public health authority of La Réunion has implemented various measures: 

  1. Management of serious cases of chikungunya in newborns and infants: The neonatology, intensive care and pediatric intensive care units at the University Hospital Center South Réunion (Centre Hospitalier Universitaire Sud Réunion) have set up a protocol for the management of severe cases of childhood chikungunya. This includes follow-up by a specialized neonatologist or neuropediatrician at 1 month, with additional follow-up visits scheduled in the event of encephalitis or neurological abnormalities.  
  2. Vector control measures: Additional vector control specialists have been hired to reinforce the capabilities of the regional health authority of La Réunion. Moreover, local vector control teams have increased the number of intervention sites on the island.  Mosquito net acquisitions are underway, including the purchase of long-lasting insecticidal mosquito nets for children and pregnant women. 
  3. Mobilization of France’s national health reserve to support HR capabilities for vector control and critical care in hospitals: Through its national health reserve, France has mobilized environmental engineers and vector control specialists to bolster La Réunion’s vector control capabilities. Further mobilization of health workers to support the island’s hospitals is expected. 
  4. Vaccination: Adults under 65 years of age with co-morbidities and professional groups at higher risk of exposure, such as vector control professionals, are vaccinated. At this stage, the vaccine is not recommended for pregnant women and individuals aged 65 years and above. The vaccination campaign began on 7 April.  
  5. Mitigation strategies at the hospital level: To mitigate the impact of a surge in chikungunya cases in hospitals, different strategies have been implemented to increase admission capacity and streamline patient flow.  
    • A triage tent has been set up in front of the University Hospital Center to identify Chikungunya patients as they arrive and redirect them towards a chikungunya-specific care pathway.
    • Additional beds have been made available.
    • Public recommendations have been issued to avoid going to emergency departments for non-critical situations, and to consult their general practitioner or contact emergency services before visiting emergency departments. 

Mayotte

  • On 26 March, the prefecture of Mayotte activated Level 2A of the ORSEC arboviruses plan following confirmation of an autochthonous chikungunya case.
  • Vector control efforts were intensified, including targeted distribution of mosquito nets and repellents, as well as insecticide and larvicide treatments around identified cases.
  • Health authorities reinforced hospital capabilities by enhancing diagnostic and laboratory capacities, strengthening in-hospital surveillance, and implementing infection prevention measures to protect staff and patients.
  • Public awareness campaigns were scaled up to inform communities about prevention measures and symptoms, and to encourage early medical consultation in suspected cases.

WHO’s response:

As part of the implementation of the Global Arbovirus Initiative, WHO is actively working with the countries to strengthen their healthcare and surveillance capacity. WHO has been supporting countries in;

  • preparedness and response to possible outbreaks, including the organization of health services 
  • implementation of effective integrated vector surveillance and control by affected countries through publishing guidelines and the provision of epidemiological surveillance materials and technical assistance to national authorities    
  • increasing the laboratory capacity, to enable timely and accurate diagnosis and case detection throughout the region 
  • delivering clinical training to health and care workers, through webinars and refresher sessions - a dedicated WHO’s Information Network for Epidemics (EPI-WIN) webinar on chikungunya was held on 7 May 2025
  • deploying experts to countries that are experiencing high magnitude outbreaks  
  • providing advice on risk assessment and risk communication. 

WHO risk assessment

Chikungunya virus is primarily transmitted by Aedes species mosquitoes and disease is characterized by an abrupt onset of fever frequently accompanied by severe joint pain and inflammation which is often very debilitating and may last for several months, or even years. Fatalities associated with infection can occur but are typically rare and occur in patients with severe chikungunya that is reported mostly in older adults with underlying medical conditions or infants infected during birth. Chikungunya virus can cause large epidemics, particularly when introduced in immunologically naive (not previously infected) human populations. These outbreaks have a high attack rate, placing a substantial demand on the public health system to conduct surveillance, case management, and differential laboratory diagnostic testing to differentiate chikungunya from illness due to co-circulating mosquito-borne viruses like dengue. Chikungunya virus can also be transmitted from viremic mothers to newborns during delivery, to laboratory workers handling chikungunya virus isolates or infected specimens, and possibly through transfusion of blood and blood products or through organ transplantation.

Chikungunya outbreaks have been documented on islands within and countries bordering the Indian and Pacific Oceans, with large populations of Aedes spp. mosquitoes, mainly, Aedes aegypti. The large outbreaks in the Indian Ocean islands in 2005-2007, however, were characterized by increased transmission by Aedes albopictus mosquitoes, which were more abundant on La Réunion and Mauritius at that time and facilitated by a viral mutation that increased transmissibility in this vector.  La Réunion is once again experiencing a large chikungunya outbreak, and Mayotte has reported locally transmitted cases for the first time in almost 20 years, indicating reintroduction of the virus.

Chikungunya can be introduced to new areas by infected travelers and may establish local transmission in the presence of vectors and a susceptible population.

The impact of the current outbreaks depends on several factors, including the current capacities for a coordinated public health response and clinical management, and the proportion of susceptible population for arboviruses. This susceptible population includes people who do not have immunity due to no previous exposure to the virus as in this situation there has been the absence of transmission for the past 20 years. Young children, elderly individuals, and those with pre-existing health conditions such as diabetes, hypertension, and cardiovascular diseases are at higher risk of developing severe disease. Additionally, people living in areas with high mosquito populations and inadequate vector control measures are at greater risk of being infected. Effective public health strategies, including vaccination, vector control, and community education, are crucial in reducing the proportion of susceptible individuals and preventing outbreaks. 

In some areas, there is a lack of medical facilities with limited geographical access, making it difficult for people to access basic health care. Other challenges can be stockouts of several essential supplies for prevention and control, lack of reagents and consumables for laboratory diagnosis, and need for re-training field teams and health workers. 

La Réunion has reported the first large-scale outbreak in nearly two decades, and the onset of autochthonous transmission in Mayotte suggest that further outbreak activity in the Indian Ocean islands can be expected, with sufficient immunologically naïve humans and Aedes vector populations to sustain human-to-mosquito-to human transmission.   

WHO advice

WHO reiterates to all countries the importance of strengthening: 1) their healthcare capacity to rapidly detect and manage cases, 2) their laboratory capacity for timely recognition and confirmation of cases; and 3) their surveillance capacity to rapidly detect trends in the incidence and implement control measures. It is relevant to maintain close monitoring of the situation in the region with active cross-border coordination and information sharing because of the possibility of cases in neighbouring countries. 

WHO encourages countries to develop and maintain the capacity to detect and confirm cases, manage patients, and implement social communication strategies to gain community support to reduce the presence of the mosquito vectors. This includes training and alerting healthcare workers on case detection and potential complications, identifying risk groups for severe disease, ensuring appropriate clinical management, and following up on cases to prevent deaths. Targeted integrated vector surveillance and control measures are essential to reduce transmission rates. 

Chikungunya can cause large outbreaks with high attack rates, affecting one-third to three-quarters of naive populations with the potential to cause heavy burdens on healthcare systems. Early detection of severe disease progression and access to proper medical attention are key to addressing clinical complications and reducing mortality.

Prevention efforts are highly focused on surveillance and control of predominantly day biting Aedes spp. mosquitoes. For protection against mosquito bites, clothing that minimizes skin exposure to mosquitoes indoors and outdoors is advised. Repellents can be applied to exposed skin or clothing in strict accordance with product label instructions.

There are currently two chikungunya vaccines that have received regulatory approvals and/or have been recommended for use in populations at risk in several countries, but the vaccines are not yet widely available nor in widespread use. WHO and external expert advisors are reviewing vaccine trial and post-marketing effectiveness and safety data in the context of global chikungunya epidemiology to inform possible recommendations for use. 

National blood services/authorities should monitor epidemiological information and strengthen haemovigilance to identify any potential transfusion transmission of chikungunya virus. Appropriate safety precautions to prevent transfusion-transmission of chikungunya virus should be taken based on the epidemiological situation and risk assessment.

No measures related to international traffic and trade are warranted at this time. 

Further information

Citable reference: World Health Organization (12 May 2025). Disease Outbreak News; Chikungunya in La Réunion and Mayotte. Available at: https://www.who.int/emergencies/disease-outbreak-news/item/2025-DON567