This is a multicenter, prospective, randomized, double-blinded, pivotal clinical study evaluating the adult dose (1 x10E4 TCID50 per 0.5 mL) of VLA1553 in comparison to a placebo control. VLA1553 and control will be administered as single immunization on Day 1. Overall, 750 male and female subjects aged 12 years to <18 years will be enrolled (i.e. ICF signed) in the study, stratified by baseline serostatus: 20% seropositive and 80% seronegative for CHIKV.

Subjects will be randomized in a 2:1 ratio to VLA1553 (n= 500) or control group (n= 250). Approximately 385 subjects will be randomized to the immunogenicity subset. Therefore, approximately 75 subjects will constitute the viremia subset.

All subjects will return to the study site at Day 8 (Visit 2), Day 29 (Visit 3), Month 3 (Day 85, Visit 4) and Month 6 (Day 180, Visit 5) for safety evaluations and immunogenicity sampling. However, immunogenicity analysis and evaluations will only be done in the immunogenicity subset. Subjects in the viremia subset will have viremia samples collected at Visits 1, 2 and 3. Samples from Visit 1 and 2 will be analyzed. The collected viremia sample of Visit 3 will only be analyzed if sample of Visit 2 is positive. In addition, for clinically indicated retrospective analysis viremia samples will be collected throughout the study from ALL subjects. Safety data collection will capture all AEs up to Month 6 (Visit 5).

After Day 180 (Visit 5), AE collection will be limited to SAEs. Subjects from the immunogenicity subset will also return to the study site at Month 12 (Day 365, Visit 6) for collection and assessment of immunogenicity samples. In addition, a clinical sample for safety laboratory evaluations will be obtained at all study visits from the immunogenicity subset only.