The epidemiology of chikungunya virus in Brazil and potential impact of vaccines

The first chikungunya virus (CHIKV) vaccine is now licensed in Brazil, the country that reports most cases globally, however, the optimal use of the vaccine remains unclear due to a poor understanding of CHIKV epidemiology and population immunity. We quantified the annual CHIKV infection and disease burden since 2014 in each of the 27 Federal Units of Brazil using a mathematical model that integrated serological surveys (N=12), confirmed chikungunya cases (N=488,234) and chikungunya deaths (N=1,719) reported between January 2014 and September 2024. Using this base, we estimated the potential impact of a vaccine over the period 2025-2029 had the population been vaccinated before the 2025 season, evaluating different rollout strategies. We found that 18.3% (95%CrI: 16.5-20.3) of the population has been infected since 2014, with the highest risk concentrated in the Northeast and Southeast. Overall, 1.13% (95%CrI: 1.07-1.19) of infections were detected by surveillance systems, with an increasing probability of symptoms with age and greater risk of symptoms in females. Vaccinating 40% of the population aged >12y (73 million doses), and assuming a vaccine efficacy of 70% against infection and 95% against disease, would avert up to 1.6 million (95%CrI: 0.5-3) cases and 198 (95%CrI: 61-359) deaths over the next five years. Despite widespread circulation, most of the country remains vulnerable to infection. CHIKV vaccination has the potential to substantially reduce disease burden.