This prospective, randomized, double-blinded, multicenter, pivotal study evaluated the final dose of VLA1553 (1 x10E4 TCID50 per dose) compared to a placebo control. The last dose of VLA1553 or control will be administered as a single immunization on Day 1. Approximately 750 male and female subjects aged 12 to <18 years will be enrolled in the study.

The pivotal immunogenicity data showed that a single-dose vaccination with VLA1553 induced a robust immune response in adolescents aged 12 to <18 years[4], confirming the excellent immunogenicity previously observed in adults[5].

Trial VLA1553-321 met its primary endpoint. VLA1553 induced levels of protective antibody titers[6] in 98.8% of participants 28 days after a single vaccination (seroresponse rate[7] of 98.8% (95% CI: 96.5, 99.8; 247 of 250 baseline seronegative participants from the per-protocol population), significantly exceeding the FDA’s requirement for study success of the lower bound of the 95%CI for SRR >70%).

The vaccine was highly immunogenic, with a Geometric Mean Titer (GMT) of 3890 in baseline seronegative participants. Neutralizing antibody GMTs at Day 29 in baseline seronegative participants were similar to GMTs observed in seropositive participants at baseline, indicating that VLA1553 induces levels of antibodies comparable to those in individuals with a history of CHIKV wild-type infection.

As reported previously, VLA1553 administered as a single dose was generally well tolerated in adolescents aged 12 to <18 years, irrespective of previous CHIKV infection, and showed a similar safety profile as reported in adults[8].

Seven hundred fifty-four individuals were vaccinated in trial VLA1553-321, and the present analysis includes data up to Day 29 (primary endpoint). An independent Data Safety Monitoring Board continuously evaluated safety data during the trial and did not identify any safety concerns. Most solicited adverse events observed following VLA1553 administration were mild or moderate and resolved within three days. Notably, the initial data suggest a favorable safety profile in seropositive participants, confirming the observations following re-vaccination of individuals in Phase 1 trial VLA1553-101[9].