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BackgroundInformation on the safety and immunogenicity of the omicron BA.4/BA.5-containing bivalent booster mRNA-1273.222 are needed. MethodsIn this ongoing, phase 2/3 trial, 50-g mRNA-1273.222 (25-g each ancestral Wuhan-Hu-1 and omicron BA.4/BA.5 spike mRNAs) is compared to 50-g mRNA-1273, administered as second boosters in adults who previously received a 2-injection (100-g) primary series and first booster (50-g) dose of mRNA-1273. The primary objectives were safety and immunogenicity 28 days post-boost. ResultsParticipants received 50-g of mRNA-1273 (n=376) or mRNA-1273.222 (n=511) as second booster doses. Omicron BA.4/BA.5 and ancestral SARS-CoV-2 D614G neutralizing antibody geometric mean titers (GMTs [95% confidence interval]) after mRNA-1273.222 (2324.6 [1921.2-2812.7] and 7322.4 [6386.2-8395.7]) were significantly higher than mRNA-1273 (488.5 [427.4-558.4] and 5651.4 (5055.7-6317.3) respectively, at day 29 post-boost in participants with no prior SARS-CoV-2-infection. A randomly selected subgroup (N=60) of participants in the mRNA-1273.222 group also exhibited cross-neutralization against the emerging omicron variants BQ.1.1 and XBB.1. No new safety concerns were identified with mRNA-1273.222. Vaccine effectiveness was not assessed in this study; in an exploratory analysis 1.6% (8/511) of mRNA-1273.222 recipients had Covid-19 post-boost. ConclusionThe bivalent omicron BA.4/BA.5-containing vaccine mRNA-1273.222 elicited superior neutralizing antibody responses against BA.4/BA.5 compared to mRNA-1273, with no safety concerns identified. (Supported by Moderna; ClinicalTrials.gov Identifier NCT04927065)