This is an observational study with primary data collection, which will combine a prospective safety cohort study and an SCRI study.

This prospective safety cohort study will be conducted in selected Brazilian municipalities participating in a pilot vaccination strategy. A total of 5,000 participants will be recruited. The primary objective is to estimate the incidence rates of a predefined set of AEs which constitute safety concerns. The secondary objectives are to measure the relative risk associated with VLA1553 administration for a predefined set of AEs which constitute safety concerns, and to assess the frequency of occurrence of any collected AE which are medically attended or meet the criteria of seriousness This study also aims to investigate VLA1553's safety in underrepresented populations by collecting all AEs, not just AEs which constitute safety concerns.

Inclusion Criteria:

1. Male or female participants who received a single dose of VLA1553 at a vaccination unit while participating in the pilot vaccination strategy.
2. Individuals that are target for the pilot vaccination strategy at the time of vaccination.
3. Individuals understanding and being able to comply with all scheduled follow-up calls/contacts and study procedures.
4. Have given informed consent/assent prior to entering the study.

Exclusion Criteria:

1. Individuals who have received investigational products within the 30 days prior to VLA1553 vaccination, or individuals who have received VLA1553/IXCHIQ previously or any authorized or approved CHIKV vaccine.
2. Individuals residing outside of the study municipalities or are not expected to remain in the study municipalities for the next 24 weeks after vaccination.
3. Pregnant women: women exposed to VLA1553 anytime during their pregnancy or within 30 days preceding their last menstrual period will be invited to participate in a separate pregnancy surveillance study (study identifier VLA1553-403).

This study will collect data from approximately 5,000 individuals who received VLA1553, as per label, as part of the pilot vaccination strategy in selected municipalities in Brazil.