The new candidate, messenger RNA (mRNA)-1273.351, encodes the prefusion stabilized spike protein of the SARS-CoV-2 variant B.1.351, first identified in South Africa, and is being assessed as a booster vaccine to increase the response to emerging SARS-CoV-2 variants. The company is also investigating a second, multivalent candidate, mRNA-1273.211, that is a combination of mRNA-1273 and mRNA-1273.351.

Through a phase II clinical trial amendment, the company will enroll 60 participants previously vaccinated with mRNA-1273 to receive a single booster dose of either 20 or 50 µg of mRNA-1273.351 or 50 micrograms of mRNA-1273.211. Through a separate amendment, participants are also being offered a booster dose of 50 µg of mRNA-1273.

Concurrently, the U.S. National Institute of Allergy and Infectious Diseases is planning to conduct a phase I trial to assess the monovalent and multivalent modified mRNA-1273 vaccines as a primary series in naïve individuals, and as a booster vaccine in those previously vaccinated with mRNA-1273.