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BACKGROUND: The first chikungunya virus (CHIKV) vaccine is now licensed in Brazil, the country that reports the most cases of CHIKV globally; however, the optimal use of the vaccine remains unclear owing to a poor understanding of CHIKV epidemiology and population immunity. We aimed to combine the distribution of cases and deaths reported since 2014 with seroprevalence studies to inform mathematical models that estimate the underlying rates of infection by state and year, and the underlying patterns of disease and death by age and sex. METHODS: We quantified the annual CHIKV infection and disease burden between 2014 and 2024 in each of the 27 federative units of Brazil using a mathematical model in a Bayesian framework that integrated serological surveys (n=12) and confirmed CHIKV disease cases (n=488 234) and CHIKV deaths (n=1719) reported between January, 2014, and September, 2024. Using this base, we estimated the potential impact of a vaccine over the period 2025-29 had the population been vaccinated before the 2025 season, evaluating different roll-out strategies. FINDINGS: We found that 18·3% (95% credible interval 16·5-20·3) of the Brazilian population has been infected since 2014, with the highest risk concentrated in the northeast and southeast. Overall, 1·13% (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 older than 12 years (73 million doses), and assuming a vaccine efficacy of 70% against infection and 95% against disease, would avert up to 1·6 million (0·5-3) cases and 198 (61-359) deaths over the next 5 years. INTERPRETATION: Despite widespread circulation, most of Brazil remains susceptible to infection. CHIKV vaccination has the potential to substantially reduce disease burden. FUNDING: CEPI. TRANSLATION: For the Portuguese translation of the abstract see Supplementary Materials section.
Cortes-Azuero et al. (Thu,) studied this question.