On May 5, 2021, Moderna announced initial data from its Phase 2 study showing that a single 50 ﾵg dose of mRNA-1273 or mRNA-1273.351 given as a booster to previously vaccinated individuals increased neutralizing antibody titer responses against SARS-CoV-2 and two variants of concern, B.1.351 (first identified in South Africa) and P.1 (first identified in Brazil).

A booster dose of mRNA-1273.351, the Companyﾒs strain-matched booster, achieved higher neutralizing antibody titers against the B.1.351 variant of concern than a booster dose of mRNA-1273. A manuscript describing these preliminary results was submitted as a preprint to medRxiv.