Age-specific chikungunya outbreak response immunisation strategies in Brazil: a modelling study

Background: Two chikungunya vaccines, Ixchiq and Vimkunya are licensed. In April 2025, Brazil is the first endemic country to license Ixchiq, but optimal age groups for vaccination remain unclear. Our aim is to model the public health impact of age-specific chikungunya outbreak response immunisation strategies in Brazil and infer broader implications for vaccine use case scenarios in outbreak prone regions. Methods: We developed an age-structured transmission dynamic model calibrated with state-level Brazilian surveillance data for 2022 and long-term average annual force of infections. We simulated outbreak response immunisation strategies targeting ages 1–11, 12–17, 18–59, and ≥60 years for Ixchiq and Vimkunya across 11 out of 27 states in Brazil. We assessed vaccine impact by symptomatic cases, deaths, and disability-adjusted life years (DALYs) averted and number needed to vaccinate (NNV) based on vaccine protection against disease only and against both disease and infection. Findings: Ixchiq and Vimkunya showed similar vaccine impact. Across strategies, vaccinating children 1–11 years yielded the lowest NNV for both vaccines, whereas vaccinating adults 18–59 years achieved the greatest absolute reduction in symptomatic cases, averting 62.5% (95% Uncertainty Intervals [UI]: 54.2–84.1) of total symptomatic cases with Vimkunya and 66.2% (58.2–86.0) with Ixchiq, under disease and infection blocking mechanism. Vaccinating adults 18–59 years with Ixchiq or Vimkunya yielded similar efficiency, with NNVs to avert a DALY of 339 (39–3412) and 361 (40–3777) respectively, under disease and infection-blocking mechanism. Interpretation: Under current licensure, vaccinating adolescents aged 12–17 years first, followed by 18–59 years are efficient strategies, with similar NNVs for both Ixchiq and Vimkunya. If eligibility expands to younger populations, vaccinating 1–11-year age group will have relatively higher efficiency. Funding: International Vaccine Institute and Japan Agency for Medical Research and Development.