Immunogenicity and Safety of an 18-Month Booster Dose of the VLA15 Lyme Borreliosis Vaccine Candidate after Primary Immunisation in Healthy Adults in the USA: Results of the Booster Phase of a Randomised, Controlled, Phase 2 Trial

Background Incidence rates of Lyme borreliosis, a tickborne disease attributed to infection by Borrelia species, are increasing, and limitations to existing treatments potentiate the possibility of severe outcomes. Nevertheless, there are no licensed vaccines for Lyme borreliosis prevention in humans. This study investigated the immunogenicity and safety of a booster dose of VLA15, an investigational outer surface protein A (OspA)-based Lyme borreliosis vaccine that has previously shown safety and immunogenicity when administered as a primary vaccination series, following a primary VLA15 vaccination series. Methods We report the results of the booster phase of a randomised, observer-blinded, placebo-controlled, multicentre, phase 2 study that enrolled healthy adults aged 18–65 years from five US clinical study centres to receive 135 μg or 180 μg VLA15 or placebo at months 0, 2, and 6 in the main study phase. Participants who received 180 μg VLA15 in the main study phase and did not have relevant protocol deviations were eligible for the booster phase (months 18–30). Participants were randomly reassigned (2:1) to receive an intramuscular injection of a VLA15 booster or placebo 1 year after the completion of primary vaccination (month 18) via a randomisation list generated by an unmasked statistician with a block size of six. Individuals involved in data safety monitoring, rerandomisation, vaccine handling, and vaccine accountability were unmasked; the study sponsor and statisticians were only unmasked after analysis of data up to 1 month after booster administration. All other individuals remained masked throughout the booster phase. The outcomes for the booster phase were the immunogenicity (evaluated in the booster per-protocol population) and safety (evaluated for all participants who received the booster) of the booster dose up to month 30. The study is registered at ClinicalTrials.gov (NCT03970733) and is completed. Findings Between Feb 4 and March 23, 2021, 58 participants (28 men and 30 women) were screened, randomly assigned, and received VLA15 (n=39) or placebo (n=19). One participant in the placebo group was lost to follow-up. The IgG geometric mean titres for each OspA serotype (serotypes 1–6) in the VLA15 group peaked at 1 month after the booster dose (1277·0 U/mL [95% CI 861·8–1892·3] to 2194·5 U/mL [1566·8–3073·7] vs 23·6 U/mL [18·1–30·8] to 36·8 U/mL [26·4–51·3] in the placebo group [p<0·0001 for all serotypes]), remained elevated at month 24 (137·4 U/mL [95·8–196·9] to 265·8 U/mL [202·9–348·2] vs 22·3 U/mL [17·7–28·0] to 29·1 U/mL [20·8–40·6] in the placebo group; p<0·0001 for all serotypes), and declined by month 30 (54·1 U/mL [38·6–75·7] to 101·6 U/mL [77·6–133·1] vs 21·9 U/mL [18·0–26·6] to 24·9 U/mL [19·0–32·6] in the placebo group; p<0·0001 for all serotypes except serotype 1 [p=0·0006]). Solicited local adverse events were reported more frequently in the VLA15 group (35 [92%, 95% CI 79–97] of 38 participants) than the placebo group (six [32%, 15–54] of 19 participants; p<0·0001) after booster vaccination. There was no significant difference in the frequency of solicited systemic adverse events between groups (20 [59%, 42–74] of 34 participants in the VLA15 group vs six [38%, 18–61] of 16 participants in the placebo group). Related unsolicited adverse events (none severe) were reported by two (5%, 1–17) of 39 participants in the VLA15 group and none (0%, 0–17) of 19 participants in the placebo group. There were no severe solicited local or systemic adverse events or deaths during the study. Interpretation A booster dose of VLA15 is safe and induces substantial anamnestic immune responses against all six OspA serotypes. As with previously investigated OspA-based Lyme borreliosis vaccines, waning immune responses were observed with VLA15, and annual boosters might therefore be required. Funding Valneva.