Observational Study to Assess the Effectiveness of VLA1553 Vaccine in Preventing Chikungunya During a Pilot Vaccination Strategy in Brazil

Inclusion Criteria: 1. Have a CHIKV RT-PCR result registered from a municipality where the pilot vaccination strategy is implemented and is in scope of the VLA1553-402 study; 2. Are in an eligible age-group to receive the VLA1553 vaccine; 3. Have been notified in SINAN as a suspected case for CHIK or Dengue surveillance during the study period; 4. Have a sample registered in GAL that is collected within 0-5 days following the onset of CHIKV-like symptoms and is tested for CHIKV using RT-PCR; 5. Have complete data in the Brazilian MoH databases (SINAN and GAL) on age, sex, municipality of residence, sample collection date, and date of symptom onset and the healthcare facilities where the suspected case was reported. Exclusion Criteria: 1. Have inconclusive RT-PCR result for CHIKV; 2. Have a detectable vaccine virus identified via sequencing; 3. For vaccinated and CHIV RT-PCR positive individuals, have no valid sequencing result available for RT-PCR positive samples collected within 30 days post-vaccination; 4. Have the first CHIKV-like symptom onset less than 14 days from date of vaccination.