A Randomized Trial Comparing Omicron-Containing Boosters with the Original Covid-19 Vaccine mRNA-1273

BackgroundOmicron-containing bivalent boosters are available worldwide. Results of a large, randomized, active-controlled study are presented. MethodsThis phase 3, randomized, observer-blind, active-controlled trial in the United Kingdom evaluated the immunogenicity and safety of 50-{micro}g doses of omicron-BA.1-monovalent mRNA-1273.529 and bivalent mRNA-1273.214 booster vaccines compared with 50-{micro}g mRNA-1273 administered as boosters in individuals [≥]16 years. Participants had previously received 2 doses of any authorized/approved Covid-19 vaccine with or without an mRNA vaccine booster. Safety and immunogenicity were primary objectives; immunogenicity was assessed in all participants, with analysis conducted based on prior infection status. Incidence of Covid-19 post-boost was a secondary (mRNA-1273.214) or exploratory (mRNA-1273.529) objective. ResultsIn part 1 of the study, 719 participants received mRNA-1273.529 (n=362) or mRNA-1273 (n=357); in part 2, 2813 received mRNA-1273.214 (n=1418) or mRNA-1273 (n=1395). Median durations (months [interquartile range]) between the most recent Covid-19 vaccine and study boosters were similar in the mRNA-1273.529 (4.0 [3.6-4.7]) and mRNA-1273 (4.1 [3.5-4.7]) (part 1), and mRNA-1273.214 (5.5 [4.8-6.2] and mRNA-1273 (5.4 [4.8-6.2]) groups (part 2). Both mRNA-1273.529 and mRNA-1273.214 elicited superior neutralizing antibody responses against omicron BA.1 with geometric mean ratios (99% CIs) of 1.68 (1.45-1.95) and 1.53 (1.41-1.67) compared to mRNA-1273 at day 29 post-boost. Although the study was not powered to assess relative vaccine efficacy, the incidence rates/1000 person years (95% CI) of Covid-19 trended lower with mRNA-1273.529 (670.5 [528.3-839.3]) than mRNA-1273 (769.3 [615.4-950.1]) and mRNA-1273.214 (633.0 [538.1-739.7]) than mRNA-1273 (711.6 [607.5-828.5]). Sequence analysis in part 2 showed that this was driven by lower incidence of Covid-19 in the mRNA-1273.214 cohort with BA.2 and BA.4 sublineages but not BA.5 sublineages. All study boosters were well-tolerated. ConclusionThe bivalent omicron BA.1-containing booster elicited superior neutralizing antibody responses against omicron BA.1 with acceptable safety results consistent with the BA.1 monovalent vaccine. Incidence rates for Covid-19 were numerically lower in participants who received mRNA-1273.214 compared to the original booster vaccine mRNA-1273, driven by the BA.2 and BA.4 sublineages.