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In this randomized, observer-blinded, phase 2/3 study, S-268019-b (n=101), a recombinant spike protein vaccine, was analyzed for noninferiority versus tozinameran (n=103), when given as a booster ≥6 months after 2-dose tozinameran regimen in Japanese adults without prior COVID-19 infection. Interim results showed noninferiority of S-268019-b versus tozinameran in co-primary endpoints for neutralizing antibodies on day 29: geometric mean titer (GMT) (124.97 versus 109.70; adjusted-GMT ratio [95% CI], 1.14 [0.94-1.39]; noninferiority P-value, P-value, 0.0004). Both vaccines elicited anti-spike-protein immunoglobulin G antibodies, and produced T-cell response (n=29/group) and neutralizing antibodies against Delta and Omicron pseudovirus and live virus variants (n=24/group) in subgroups. Most participants reported low-grade reactogenicity on days 1-2, the most frequent being fatigue, fever, myalgia, and injection-site pain. No serious adverse events were reported. In conclusion, S-268019-b was safe and showed robust immunogenicity as a booster, supporting its use as COVID-19 booster vaccine.JRCT IDjRCT2031210470HighlightsThird COVID-19 vaccine dose (booster) enhances immune responseInterim phase 2/3 data for booster ≥6 months after the 2nd dose in Japan are shownS-268019-b was noninferior to tozinameran in inducing neutralizing antibodiesSera boosted with either vaccines neutralized Delta and Omicron virus variantsS-268019-b was safe, and results support its use as a booster in vaccinated adults