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

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).

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

Primary outcomes

• Detection of CHIKV in VLA1553-vaccinated (≥14 days after vaccination) and unvaccinated study individuals with CHIKV-like symptoms confirmed by reverse transcriptase polymerase chain reaction (RT-PCR). [Study start through study completion, an average of 23 months]

Secondary outcomes

• Detection of CHIKV in VLA1553-vaccinated (≥14 days after vaccination) and unvaccinated study individuals ≥50 years of age with CHIKV-like symptoms confirmed by RT-PCR. [Study start through study completion, an average of 23 months]

Other outcomes

• Detection of CHIKV in VLA1553-vaccinated (≥14 days after vaccination) and unvaccinated individuals with CHIKV-like symptoms confirmed by RT-PCR, stratified by sex and age group (≥18 to 59 and ≥ 60 years, depending on the age group approved for the PVS) [Study start through study completion, an average of 23 months]
• Detection of CHIKV in VLA1553-vaccinated (≥14 days after vaccination) and unvaccinated study individuals with CHIKV-like symptoms confirmed by RT-PCR, stratified by time since vaccination (from 0 to 6 months after Day 15) [Study start through study completion, an average of 23 months]
• CHIKV hospitalizations (confirmed by RT-PCR) in VLA1553-vaccinated (≥14 days after vaccination) and unvaccinated study individuals; CHIKV deaths (confirmed by RT-PCR) in VLA1553-vaccinated (≥14 days after vaccination) and unvaccinated study individuals [Study start through study completion, an average of 23 months]
• Magnitude and direction of estimated level of bias due to unmeasured pre-vaccination strategy CHIKV infection, and estimated VE corrected for this bias. [Study start through study completion, an average of 23 months]

Sex: All, ages: 18 Years to 59 Years, accepts healthy volunteers

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.