Safety and Immunogenicity of 1 or 2 Additional Doses of the Adjuvanted Recombinant Zoster Vaccine Administered 5–6 Years After Primary Vaccination in Adults ≥50 Years

Background: A phase 3b extension of the ZOE-50/70 trials evaluated long-term efficacy, immunogenicity, and safety of the recombinant zoster vaccine (RZV) in participants ≥50 years, with a 6-year follow-up after completion of the primary studies. A subset of participants was evaluated for immunogenicity and safety of 1 or 2 additional doses administered 5-6 years after primary vaccination. Methods: Participants were randomized to 1 additional dose (1-additional dose group, n = 61), 2 additional doses (revaccination group, n = 60), or no additional vaccination (control, n = 119). Humoral and cell-mediated immunity were evaluated by antiglycoprotein E (gE) antibodies, gE-specific CD4[2+] T-cells, and memory B-cells. Reactogenicity was evaluated for 7 days postvaccination and overall safety was evaluated throughout the study. NCT02723773. Results: Anti-gE geometric mean concentrations (GMCs) were 10 000-11 500 mIU/mL in all groups preadditional vaccination. Geometric mean concentrations peaked at 1 month after 1 dose (73 834.4 and 79 419.8 mIU/mL in the 1-additional dose and revaccination groups, respectively), declined at Year 1, but remained above preadditional vaccination levels thereafter. Geometric mean concentration was 64 603.0 mIU/mL 1 month after the second dose in the revaccination group. Geometric mean concentrations in the control group were 8825.4 mIU/mL at Year 1 and 6858.8 mIU/mL at Year 6. The frequency of gE-specific CD4[2+] T-cells and memory B-cells followed a similar pattern. Pain and fatigue were the most common solicited adverse events. No serious adverse events related to RZV were reported. Conclusions: A single additional RZV dose elicited strong and durable humoral and cell-mediated anamnestic responses, with a reactogenicity and safety profile as established in primary studies. Clinical Trial Registration: NCT02723773.