Pooled safety evaluation for a new single-shot live-attenuated chikungunya vaccine.

Background: Chikungunya disease, caused by chikungunya virus (CHIKV), is associated with substantial morbidity, including debilitating CHIKV-related arthralgia.

Methods: Three clinical trials of a CHIKV vaccine (VLA1553, IXCHIQ®) were conducted in the US: a Phase 1 dose-finding trial, a pivotal Phase 3 trial, and a Phase 3 lot-to-lot consistency trial. Participants were healthy adults (≥18 years) and received a single intramuscular dose of VLA1553 (3520 participants) or placebo (1033 participants). Solicited injection site and systemic adverse events (AEs) (10 to 14 days post-vaccination), unsolicited AEs (28 and 180 days post-vaccination), AEs of special interest (AESIs) (28 days post-vaccination), medically attended AEs (MAAEs), serious AEs (SAEs) (180 days post-vaccination), and pregnancies were evaluated. Safety data were pooled, and analyses were descriptive.

Results: Overall, 63.7% of participants receiving VLA1553 experienced AEs, (44.7% for placebo), that were generally mild. Solicited injection site AEs, solicited systemic AEs, and unsolicited (Day 29) AEs were reported by 15.5%, 50.9%, 22.7% of participants who received VLA1553 and 11.1%, 26.9%, 13.4% who received placebo. Arthralgia was reported by 16.7% of participants who received VLA1553 and 4.8% of participants who received placebo; none required medical attention. MAAEs, AESIs, and SAEs were reported by 12.4%, 0.3%, 1.5% of participants who received VLA1553 and 11.3%, 0.1%, 0.8% of participants who received placebo. Protocol-defined AESIs were mild and short-lived, and two VLA1553-related SAEs resolved without sequelae. There were no clinically important differences in AE incidence based on age or medical history, and no VLA1553-related adverse pregnancy outcomes. There were 3 deaths (2 in the VLA1553 group and 1 in the placebo group), none was vaccine-related.

Conclusions: A single dose of VLA1553 presented with an excellent local tolerability profile and overall safety in line with that expected for a live-attenuated vaccine. The safety profile was comparable in participants aged 18-64 years and ≥65 years.