Chikungunya virus-neutralizing antibody persistence four years after single-dose vaccination with VLA1553 (IXCHIQ®).

VLA1553 is a live-attenuated chikungunya virus vaccine approved for active immunization, in the EU, Brazil, Canada and the UK, as a prophylactic measure for individuals travelling to or living in areas where the chikungunya virus is circulating. Due to the unpredictable epidemic occurrence of chikungunya, a 50% micro-plaque reduction neutralization test titre (μPRNT50) of ≥150, was previously accepted by regulators as an immunological surrogate endpoint that is reasonably likely to predict clinical benefit. At this point in time, this phase 3b open-label, single-arm long-term focuses on antibody persistence in a subset (N = 363) of VLA1553 vaccinees from a pivotal phase 3 trial (Schneider et al., 2023) where 4115 adult participants received VLA1553 or placebo. At Year 4, 254 participants remained in follow-up with slightly more females than males. The population is predominantly White with no Hispanic or Latino background. The seroresponse rate was 94.5% (240/254, 95% CI 90.9% to 97.0%). The Day 29 GMT for the long-term follow-up cohort was 3542, and remained high with 1056 at Year 1, 785 at Year 2, 737 at Year 3 and 589 at Year 4, considerably exceeding the seroresponse threshold of 150. In adults aged ≥65 years, antibody persistence was similar to younger adults throughout the follow-up. Single dose vaccination with our live-attenuated chikungunya virus vaccine VLA1553 resulted in persistent neutralizing antibody titers well above the defined seroresponse threshold for at least 4 years.