Immunogenicity and safety of a 4 th homologous booster dose of a SARS-CoV-2 recombinant spike protein vaccine (NVX-CoV2373): a phase 2, randomized, placebo-controlled trial

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Summary Background The emergence of SARS-CoV-2 variants has significantly reduced the efficacy of some approved vaccines. A fourth dose of NVX-CoV2373 (5µg SARS-CoV-2 rS + 50µg Matrix-M™ adjuvant) was evaluated to determine induction of cross-reactive antibodies to variants of concern. Methods A phase 2 randomized study assessed a fourth dose of NVX-CoV2373 in adults 18–84 years of age (2-dose primary series followed by third and fourth doses at 6-month intervals). Local/systemic reactogenicity was assessed the day of vaccination and for 6 days thereafter. Unsolicited adverse events (AEs) were reported. Immunogenicity was measured before, and 14 days after, fourth dose administration using anti-spike neutralization assays against the ancestral SARS-CoV-2 strain and Omicron sublineages. Antigenic cartography assessed antigenic distances between ancestral and variant strains. Results Among 1283 enrolled participants, 258 were randomized to receive the 2-dose primary series, of whom 104 received a third dose, and 45 received a fourth dose of NVX-CoV2373. The incidence of local/systemic reactogenicity events increased after the first three doses of NVX-CoV2373, and leveled off after dose four. Unsolicited AEs were reported in 9% of participants after dose 4 (none severe or serious). Neutralization antibody titers increased following booster doses. Antigenic cartography demonstrated reductions in antigenic distance between ancestral and variant SARS-CoV-2 strains with increased number of NVX-CoV2373 doses. Conclusions A fourth dose of NVX-CoV2373 enhanced immunogenicity without increasing reactogenicity. Antigenic cartography demonstrated a more universal-like response against SARS-CoV-2 variants after a fourth dose of NVX-CoV2373, indicating that updates to the vaccine composition may not be warranted. Trial registration number NCT04368988

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