Assessment of Chikungunya Virus Seroprevalence Before VLA1553 Vaccination in the Municipalities Selected for Participation in the VLA1553 Pilot Vaccination Strategy 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.

• Live-attenuated CHIKV vaccine VLA1553 (Biological): Non-interventional study: VLA1553 is used in the pilot vaccination strategy which is carried out after this observational study.

Primary outcomes

• To prospectively evaluate the association between vaccination uptake and previous CHIKV exposure (pre-existing immunity to CHIKV infection) after implementing a pilot vaccination in the age-group approved per label. [on Day 1]

Other outcomes

• To evaluate CHIKV seroprevalence in the selected municipalities before the implementation of a pilot vaccination in the population ≥2 years of age [on Day 1]
• The association, measured as an odds ratio, between vaccination uptake and prevaccination strategy CHIKV seropositivity (IgG) [on Day 1]
• Pre-vaccination strategy CHIKV seropositivity (IgG) stratified by vaccination status with VLA1553 vaccine, among the population approved per label [on Day 1]
• Vaccination uptake, measured as cumulative incidence, according to prevaccination strategy CHIKV seropositivity (IgG) [on Day 1]
• CHIKV seropositivity (IgG) among the study participants ≥2 years of age assessed by rapid diagnostic test (RDT) or Enzyme-Linked Immunosorbent Assay (ELISA) tested on a dried blood spot (DBS) sample before the implementation of the VLA1553 PVS [on Day 1]

Sex: All, ages: 2 Years to any, accepts healthy volunteers

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.