Lyme borreliosis is the most common tick-borne disease in temperate climates of the northern hemisphere. Although in some cases Lyme borreliosis progresses to serious outcomes, no human vaccines are available for its prevention. A previous report showed positive immunogenicity and safety of VLA15, an investigational Lyme borreliosis vaccine based on Borrelia burgdorferi outer surface protein A (OspA), when administered as a 0-2-6-month or 0-6-month primary series to children, adolescents, and adults. Here, we report data from the same trial after receipt of an initial booster dose at month 18.

This ongoing, randomised, observer-blind, placebo-controlled, phase 2 trial is taking place at 14 clinical study centres in Lyme borreliosis-endemic areas in the USA. Healthy, eligible participants aged 5–65 years were enrolled. Participants were randomly assigned within each age cohort (18-65 years, 12-17 years, and 5-11 years) in a 1:1:1 ratio to receive intramuscular injections of VLA15 at months 0, 2, and 6; VLA15 at months 0 and 6 and placebo at month 2; or placebo at months 0, 2, and 6. In this phase of the trial, a month 18 VLA15 booster vaccination was administered to participants in both VLA15 groups (termed VLA15 M0-2-6-18 and VLA15 M0-6-18, respectively); the placebo group received placebo at month 18. Data up to month 12, including the primary endpoints, have been reported previously; this report includes data up to month 19. Secondary endpoints related to the month 18 booster vaccination included OspA serotype-specific IgG geometric mean titres (GMTs; evaluated in the per-protocol analysis set) and solicited and unsolicited adverse events (evaluated in the safety analysis set [ie, all participants who received one or more vaccinations]) from month 18 to month 19. This trial is registered at ClinicalTrials.gov, NCT04801420, and is closed for recruitment.

Between March 15, 2021, and Feb, 24, 2022, 625 participants were randomly assigned and received the first vaccination. 532 (85%) of 625 participants were White, 68 (11%) were Black or African American, 13 (2%) were Asian, one (<1%) was an American Indian or Alaska Native, and 11 (2%) had their race recorded as other. 321 (51%) of 625 participants were female and 304 (49%) were male. The month 18 booster vaccination was administered to 449 participants at 13 of the trial sites (148 participants in the VLA15 M0-2-6-18 group; 143 participants in the VLA15 M0-6-18 group; 12 other VLA15 recipients; and 146 participants in the placebo group) between Sept 21, 2022, and Jan 24, 2023. The 12 other VLA15 booster recipients, of whom 11 received VLA15 and one received placebo for the month 18 booster, received at least one primary or booster VLA15 dose but could not be evaluated within a designated VLA15 group for safety because of missed or incorrect vaccinations, and were excluded from post-booster immunogenicity analyses. Of the 513 participants included in the per-protocol analysis set, 398 received the booster and 394 completed the month 19 visit. OspA-specific IgG GMTs in both VLA15 groups declined after the primary series up to month 18. At 1 month after the month 18 booster vaccination, GMTs in both VLA15 groups rose to levels that exceeded those after the primary series, ranging in the overall population from 1057·0 U/mL (95% CI 843·1–1325·1; serotype 1) to 1807·9 U/mL (1486·2–2199·3; serotype 2) in the M0-2-6-18 group and from 830·0 U/mL (621·3-1108·9; serotype 1) to 1603·1 U/mL (1239·7–2073·0; serotype 2) in the M0-6-18 group. GMTs at month 19 were higher in the paediatric cohorts compared with adults, consistent with observations after the primary vaccination series. The tolerability profile of the month 18 booster was similar to that of the primary doses and generally similar across age cohorts. Related unsolicited adverse events were reported by four (1%) of 302 VLA15 booster vaccination recipients and three (2%) of 147 placebo booster recipients within 1 month after the month 18 booster; all of these events resolved without sequelae. Unsolicited adverse events leading to trial withdrawal, unsolicited serious adverse events, and adverse events of special interest that were reported up to month 19 were all considered unrelated to trial vaccination and occurred before the month 18 booster. No deaths were reported up to month 19 of the trial.

The safety and robust anamnestic immune responses associated with VLA15 boosting support its use as a strategy to increase anti-OspA antibody levels before tick season among children, adolescents, and adults. Forthcoming data after administration of subsequent annual boosters will provide further information about VLA15 antibody persistence and boostability.

Valneva and Pfizer.