Safety and immunogenicity of a booster dose of S-268019-b: Interim findings of a Phase 3, open-label clinical study in Japan

•S-268019-b is a protein-based subunit vaccine with a squalene-based adjuvant.•Japanese adults and elderly with prior mRNA vaccination received S-268019-b booster.•S-268019-b booster dose was well tolerated and enhanced immunity against SARS-CoV-2.•It also elicited neutralizing antibodies against Omicron and T-cell responses. Despite the initial success of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccines in prevention of symptomatic and severe diseases, booster vaccination has become increasingly important with the advent of variants with immune-escaping capacity. Herein, we report the safety and immunogenicity of S-268019-b, comprising SARS-CoV-2 spike protein and a squalene-based adjuvant, as a booster dose. We performed an interim analysis of an open-label, Phase 3 study data until Day 29 following S-268019-b booster in Japanese adults (aged 20–64 years) who had completed primary vaccination with mRNA-1273 and in Japanese elderly (aged ≥ 65 years) who had completed primary vaccination with mRNA-1273 or BNT162b2. Reactogenicity was mild in most participants; no serious treatment-related adverse events were noted. S-268019-b enhanced SARS-CoV-2 neutralizing antibodies, immunoglobulin G antibodies, and predominant T-helper 1-mediated immune reaction in all cohorts, regardless of age, in Japanese participants with prior vaccination with mRNA vaccines.