Immunogenicity and Safety of Booster Dose of S-268019-b or Tozinameran in Japanese Participants: An Interim Report of Phase 2/3, Randomized, Observer-Blinded, Noninferiority Study

In this randomized, observer-blinded, phase 2/3 study, we assessed S-268019-b, a recombinant spike protein vaccine. We analyzed the noninferiority of S 268019-b (n=103), versus tozinameran (n=103), when given as a booster [≥]6 months after the 2-dose tozinameran regimen in Japanese adults without prior COVID-19 infection. Interim results showed that S-268019-b was noninferior to tozinameran in co-primary endpoints: geometric mean titer (GMT) (126.42 versus 108.20; adjusted-GMT ratio [95% CI], 1.17 [0.96-1.42]; noninferiority P-value, <0.0001) and seroresponse rate (both 100%; noninferiority P-value, 0.0004) for neutralizing antibodies on day 29. Both vaccines elicited anti-spike protein immunoglobulin G antibodies, and produced T-cell response (n=30/group) and neutralized Delta and Omicron pseudovirus variants (n=24/group) in subgroups. Most participants reported low-grade reactogenicity on days 1 and 2, the most frequent being fatigue, fever, myalgia, and injection-site pain. No serious adverse events were reported. S-268019-b safety and robust immunogenicity as a booster, supporting its use as COVID-19 booster vaccine. Keywords: booster, clinical trial, neutralization test, SARS-CoV-2, recombinant spike protein, immunization JRCT ID: jRCT2031210470