The trial enrolled 11,454 participants aged 12 years or older across the U.S., United Kingdom, and Canada from March 28 to August 23, 2023. Participants were randomly assigned to receive a 10 µg dose of mRNA-1283.222 or a 50 µg dose of mRNA-1273.222. Both bivalent vaccines targeted the original SARS-CoV-2 strain (D614G) and omicron BA.4/BA.5 variants.
Among 11,417 vaccinated participants, 560 of 5,679 in the mRNA-1283 group (9.9%) and 617 of 5,687 in the mRNA-1273 group (10.8%) developed confirmed COVID-19 through January 31, 2024. The relative vaccine efficacy (rVE) of mRNA-1283 compared to mRNA-1273 was 9.3%, meeting the non-inferiority threshold of –10%.
In participants aged 50 years and older, rVE was 15.0%. In those 65 and older, rVE was 13.5%. For severe COVID-19, rVE was 38.1% based on 55 events through January 31, 2024.
At day 29, the geometric mean concentration of neutralizing antibodies against BA.4/BA.5 was 2,346.2 in the mRNA-1283 group and 1,753.8 in the mRNA-1273 group. For D614G, the geometric mean concentrations were 10,657.6 and 8,576.5, respectively.
The geometric mean ratios favored mRNA-1283: 1.3 for BA.4/BA.5 and 1.2 for D614G. The seroresponse rate for BA.4/BA.5 was 79.9% with mRNA-1283 and 65.5% with mRNA-1273. For D614G, seroresponse rates were 83.6% versus 72.9%.
Local and systemic adverse event rates were comparable. Injection-site pain was reported in 68.5% of mRNA-1283 recipients and 77.5% of mRNA-1273 recipients. Fatigue, headache, and myalgia were the most common systemic reactions. The majority of reported adverse reactions were classified as mild or moderate in severity.
One participant in the mRNA-1273 group with underlying cardiovascular disease died 7 days after vaccination; the event was assessed as related by the investigator due to temporal association.
The study met its prespecified endpoints for efficacy and immunogenicity, supporting the use of mRNA-1283 as a potential COVID-19 booster.
Full disclosures can be found in the study.
Source: The Lancet Infectious Diseases