Combined immunogenicity evaluation for a new single-dose live-attenuated chikungunya vaccine

Background: Chikungunya is a serious and debilitating viral infection with a significant disease burden. VLA1553 (IXCHIQ®) is a live-attenuated vaccine licensed for active immunization for prevention of disease caused by chikungunya virus (CHIKV).

Methods: Immunogenicity following a single dose of VLA1553 was evaluated in healthy adults aged ≥18 years in two Phase 3 trials [N = 656 participants (per protocol analysis set)]. Immunogenicity data to 180 days post-vaccination [geometric mean titres (GMTs), seroresponse rate, seroconversion rate] were pooled for the two trials. A comparison of subgroups based on age, sex, body mass index (BMI), race and baseline seropositivity was included. All analyses were descriptive.

Results: Most participants were aged 18-64 years (N = 569/656 [86.7%]), there were slightly more females (N = 372/656 [56.7%]), most were not Hispanic/Latino (N = 579/656 [88.3%]), and most were White (N = 517/656 [78.8%]). In baseline seronegative participants, GMT peaked at Day 29 post-vaccination, and subsequently declined slightly but remained elevated until Day 180. At Days 29, 85 and 180, seroresponse rate was 98.3, 97.7 and 96.4% and seroconversion rate was 98.5, 98.4 and 98.2%. There were no differences in seroresponse rate in participants aged 18-64 years or ≥65 years at Day 29 (98.1 vs 100%), Day 85 (97.4 vs 100%) and Day 180 (96.3 vs 96.5%) nor based on sex, BMI, ethnicity or race. An immune response was shown in a small heterogenous population of baseline seropositive participants, with GMTs showing the same trend as baseline seronegative participants.

Conclusions: A single dose of VLA1553 elicited a very strong immune response by Day 29 that remained elevated at Day 180 in both baseline seronegative and seropositive participants in a combined evaluation of two Phase 3 trials. The vaccine was similarly immunogenic in participants aged ≥65 years and 18-64 years, and there were no differences based on subgroup analyses for sex, BMI, ethnicity or race.