This is an observational, non-interventional, test-negative case-control (TNCC) study to estimate the vaccine effectiveness of VLA1553 against Chikungunya virus in a real-world setting.

The objective of this study is to evaluate the effectiveness of vaccination with VLA1553 in the prevention of laboratory-confirmed symptomatic cases of CHIK after a single dose of VLA1553 administered as part of a pilot vaccination strategy following licensure in Brazil.

The study will commence following the implementation of the pilot vaccination strategy in selected municipalities. Data for the analyses will be obtained from official Brazilian Ministry of Health surveillance systems, specifically the GAL (Laboratory Environment Management System), SINAN (Notifiable Diseases Information System), and Si-PNI (National Immunization Program Information System).

Inclusion Criteria:

1. Have place of residence in a municipality where the pilot vaccination strategy is implemented;
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 the first CHIKV-like symptom onset less than 14 days from date of vaccination.

Residents of the selected areas, within the age limits approved for the PVS, presenting at the health care facilities during the study period.