Immunogenicity and safety of a monovalent Omicron XBB.1.5 SARS-CoV-2 recombinant spike protein vaccine in previously unvaccinated, SARS-CoV-2 seropositive participants: Primary day-28 analysis of a phase 2/3 open-label study

Background: Most of the population has been infected with SARS-CoV-2 and, thus, is primed by natural exposure. As such, it was assessed whether a single dose of the monovalent XBB.1.5 vaccine, NVX-CoV2601, elicited a comparable immune response to XBB.1.5 in seropositive unvaccinated participants to that in previously vaccinated participants, thereby allowing the former to forego a two-dose primary series. Methods: In this phase 2/3, open-label, single-arm study (2019nCoV-313/NCT05975060 [group 2]), vaccine-naive participants >= 18 years with previous SARS-CoV-2 infection received one dose of NVX-CoV2601. This analysis compared the 28-day immunogenicity and safety of NVX-CoV2601 in vaccine-naive and previously vaccinated (>= 3 prior mRNA-based vaccines, from 2019nCoV-313 group 1) participants. Noninferiority of neutralizing antibody (nAb) response in vaccine-naive versus vaccinated participants was the primary objective. The day-28 geometric mean titer (GMT) ratio (GMTR) and seroresponse rate (SRR; percentage of participants with a >= 4-fold rise in antibody response from baseline) were measured, and safety was assessed. Results: Of the participants enrolled from September 11 to November 15, 2023, per-protocol sets included 306/ 338 (90.5%) vaccine-naive and 309/332 (93.1%) vaccinated participants. At day 28, adjusted GMTs (95% CI) against XBB.1.5 in the vaccine-naive and vaccinated groups were 1491.5 (1277.5-1741.4) and 841.4 (723.9-978.0), respectively. The vaccine-naive-vaccinated nAb GMTR was 1.8 (95% CI 1.43-2.20) and SRRs were 74.3% and 64.3% for vaccine-naive and vaccinated participants, respectively (SRR difference: 10.0 [95% CI 2.6-17.4]). No new safety signals or events of special interest were reported. Conclusions: A single dose of NVX-CoV2601 in vaccine-naive participants with a history of SARS-CoV-2 infection elicited a robust neutralizing antibody response that was noninferior to that observed in vaccinated participants. The vaccine was well-tolerated. These data support the use of NVX-CoV2601 as a single dose, regardless of prior vaccination history. Trial registration: NCT05975060.