Chikungunya virus infection can lead to long-term debilitating symptoms. A precursor phase 3 clinical study showed high seroprotection (defined as a 50% plaque reduction of chikungunya virus-specific neutralising antibodies on a micro plaque reduction neutralisation test [μPRNT] titre of ≥150 in baseline seronegative participants) up to 6 months after a single vaccination of the chikungunya virus vaccine VLA1553 (Valneva Austria, Vienna, Austria) and a good safety profile. Here we report antibody persistence and safety up to 2 years.

In this single-arm, multicentre, phase 3b study, we recruited participants from the precursor phase 3 trial from professional vaccine trial sites in the USA. Participants (aged ≥18 years) were eligible if they had completed the previous study and received VLA1553. Chikungunya virus-specific neutralising antibodies were evaluated at 28 days, 6 months, and 1 year and 2 years after vaccination. The primary outcome was the proportion of seroprotected participants (ie, μPRNT₅₀ titre of ≥150) at 1 and 2 years, assessed in all eligible participants who had at least one post-vaccination immunogenicity sample available, overall and by age group at the time of vaccination (18–64 years and ≥65 years). Adverse events of special interest at the time of transition from the previous study to the current study (ie, at 6 months) and serious adverse events during the current study were recorded (ie, between 6 months and 2 years). All analyses were descriptive. This study is registered with ClinicalTrials.gov, NCT04838444, and immunogenicity follow-up is ongoing.

In the precursor study, participants were screened between Sept 17, 2020, and April 10, 2021; data cutoff for this analysis was March 31, 2023. Of 2724 participants in the precursor study who received one dose of VLA1553, 363 participants were analysed in this study (310 [85%] aged 18–64 years and 53 [15%] aged ≥65 years at enrolment in the precursor study; mean age 47·7 years [SD 14·2], 207 [57%] of 363 participants were female, 156 [43%] were male, 280 [77%] were White, and 314 [87%] were not Hispanic or Latino). Strong seroprotection was observed at 1 year (98·9% [356 of 360 assessable participants; 97·2–99·7]) and 2 years (96·8% [306 of 316; 94·3–98·5]) after vaccination, and was very similar between those aged 18–64 years (at 1 year: 98·7% [303 of 307; 96·7–99·6]; at 2 years: 96·6% [256 of 265; 93·7–98·4]) and those aged 65 years and older (at 1 year: 100% [53 of 53; 93·3–100]; at 2 years: 98·0% [50 of 51; 89·6–100]) at each timepoint. No adverse events of special interest were ongoing at the time of transition. Ten serious adverse events occurred in nine (2%) participants between the 6-month and 2-year timepoints, including one death (due to drug overdose) that was determined to not be related to VLA1553.

After a single VLA1553 vaccination, chikungunya virus-neutralising antibodies above the threshold considered to be protective persisted up to 2 years and there were no long-term serious adverse events related to vaccination. VLA1553 is an efficient and safe intervention that offers high seroprotection against chikungunya virus infection, a virus likely to spread globally with an urgent demand for long-lasting prophylaxis.

Valneva Austria, Coalition for Epidemic Preparedness Innovation, and EU Horizon 2020.