ABSTRACTBackgroundThis interim analysis of an ongoing phase I randomized clinical trial evaluated the safety, reactogenicity, and immunogenicity of mRNA-1283, a next-generation SARS-CoV-2 messenger RNA (mRNA)-based vaccine encoding 2 segments of the spike protein (ie, receptor binding and N-terminal domains).MethodsHealthy aged adults 18-55 years (n = 104) were randomized (1:1:1:1:1) to receive 2 doses of mRNA-1283 (10, 30, or 100 μg) or mRNA-1273 (100 μg) administered 28 days apart, or a

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... dose of mRNA-1283 (100 μg). Safety was assessed and immunogenicity was measured by serum neutralizing antibody (nAb) or binding antibody (bAb) responses.ResultsAt the interim analysis, no safety concerns were identified and no serious adverse events, adverse events of special interest, or deaths were reported. Solicited systemic adverse reactions were more frequent with higher dose levels of mRNA-1283 than with mRNA-1273. At day 57, all dose levels of the 2-dose mRNA-1283 regimen (including the lowest dose level [10 μg]) induced robust nAb and bAb responses that were comparable to those of mRNA-1273 (100 μg).ConclusionsmRNA-1283 was generally safe in adults, with all dose levels of the 2-dose regimen (10, 30, and 100 μg) eliciting similar immunogenicity as the 2-dose mRNA-1273 regimen (100 μg).Clinical Trials RegistrationClinicaltrials.gov,NCT04813796