This is a cross-sectional serosurvey using household cluster sampling conducted before the VLA1553 pilot vaccination strategy will be implemented in about 10 municipalities in Brazil.

Households will be selected randomly using a multi-stage random sampling at the census tract, street, and household levels, to select study participants. The study aims to prospectively evaluate the association between vaccination uptake and previous CHIKV exposure (pre-existing immunity to CHIKV infection) in the age-group approved per label in Brazil (e.g., adults [≥18 years of age] or individuals ≥12 years of age). Seroprevalence among vaccinated and unvaccinated individuals will be used to parameterize a probabilistic bias analysis conducted as part of the VE study. Finally, the study will explore the prevalence of CHIKV exposure in the different study communities before the VLA1553 pilot vaccination strategy in individuals ≥2 years of age.

Inclusion Criteria:

1. Individuals residing in the selected municipalities of Brazil where the VLA1553 pilot vaccination strategy will be implemented.
2. Individuals ≥2 years of age
3. Individuals who sleep 3 nights a week or more in a household that is selected by cluster sampling.

Exclusion Criteria:

1. Individuals or their legally accepted representatives (LAR) who are not willing or not able to provide informed consent (and assent, as required) according to country-specific procedures.

Study population will consist of individuals ≥2 years of age residing in the municipalities where the pilot VLA1553 vaccination strategy will take place and who meet the eligibility criteria