The authors describe a phase 1, dose-escalation, open-label trial of an mRNA-1273 candidate vaccine encoding the stabilised pre-fusion SARS-CoV-2 spike protein. 45 healthy adults received two vaccinations, 28 days apart, with mRNA-1273 in a dose of 25 μg, 100 μg or 250 μg. There were 15 participants in each dose group. After the first vaccination, antibody responses were found to be higher with the higher dose (day 29 ELISA anti-S-2P antibody geometric mean titre [GMT] 40,227 in the 25 μg group, 109,209 in the 100 μg group and 213,526 in the 250 μg group). Following the second vaccination, the titres increased (day 57 GMT 299,751, 782,719 and 1,192,154, respectively). After the second vaccination, serum-neutralising activity was detected by two methods in all participants assessed, with values similar to those in the upper half of the distribution of a panel of control convalescent serum specimens. Adverse events occurred in more than half of the participants and included: fatigue, chills, headache, myalgia and pain at the injection site. Systemic adverse events were more common after the second vaccination, particularly with the highest dose, and 21% of participants in the 250 μg dose group reported one or more severe adverse events. Therefore the mRNA-1273 vaccine induces anti-SARS-CoV-2 immune responses in all participants, with no trial-limiting safety concerns seen.
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