We previously presented day 29 interim safety and immunogenicity results from a phase 2/3 study (NCT04927065) comparing the Omicron-BA.1-containing bivalent vaccine mRNA-1273.214 (50-µg) to the 50-µg mRNA-1273 booster in adults who previously received the mRNA-1273 primary series (100-µg) and mRNA-1273 first booster (50-µg) dose. Primary endpoints were safety, non-inferiority of the neutralizing antibody (nAb) and seroresponse against Omicron BA.1, superiority of the nAb response against Omicron-BA.1, and non-inferiority of the nAb response against ancestral SARS-CoV-2 for second boosters of mRNA-1273.214 versus mRNA-1273 at days 29 and 91. The key secondary endpoint was the seroresponse difference of mRNA-1273.214 versus mRNA-1273 against ancestral SARS-CoV-2 at days 29 and day 91. Participants were sequentially enrolled and dosed with 50-µg of mRNA-1273 (n = 376) or mRNA-1273.214 (n = 437) as second booster doses. Here we present day 91 post-booster results. In participants with no pre-booster, severe acute respiratory syndrome coronavirus 2-infection (SARS-CoV-2), mRNA-1273.214 elicited Omicron-BA.1-nAb titers (95% confidence interval [CI]) that were significantly higher (964.4 [834.4-1114.7]) than those of mRNA-1273 (624.2 [533.1-730.9]) and similar to those of mRNA-1273 against ancestral SARS-CoV-2 at day 91. mRNA-1273.214 also induced higher binding antibody responses against Omicron BA.1 and alpha, gamma and delta variants than mRNA-1273. Safety profiles were similar for both vaccines. The Omicron-BA.1 bivalent vaccine improved antibody responses compared to mRNA-1273 through 90 days post-booster.
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