Abstract
Human vaccines against Lyme borreliosis, the most common tick-borne illness in Northern Hemispheric temperate climates, remain unavailable. Previous evaluations demonstrated the safety and immunogenicity of primary and booster dosing with VLA15, an investigational vaccine containing six Borrelia burgdorferi sensu lato outer surface protein A serotypes. In this randomized, observer-blinded, placebo-controlled, phase 2 trial, participants 5 - 65 years of age were randomized 1:1:1 to receive VLA15 at Months (M) 0, 2, and 6; VLA15 at M0 and M6 and placebo at M2; or placebo at each time point. VLA15 and placebo recipients received VLA15 boosters and placebo, respectively, at M18 and M30. Here we report safety and immunogenicity from M19 through M31; earlier data were reported previously. In both VLA15 groups, serotype-specific immunoglobulin G geometric mean titers declined after the M18 booster but remained higher than after primary vaccination. The M30 VLA15 boosters induced robust, anamnestic immune responses across serotypes and age groups that were qualitatively similar to those after the M18 boosters. The M18 and M30 boosters had similar, acceptable tolerability profiles; no safety concerns arose through M31. These results further support the paradigm of VLA15 primary vaccination followed by yearly boosting to optimize potential protection against Lyme borreliosis. ( ClinicalTrials.gov: NCT04801420 ).