In Part 1, participants will be randomized in a 1:1 ratio to receive a single dose of either mRNA-1273.529 or mRNA-1273.~In Part 2, participants will be randomized in a 1:1 ratio to receive a single dose of either mRNA-1273.214 or mRNA-1273.~All participants will have previously received 2 or 3 doses of an authorized/approved COVID-19 vaccine. Participants who previously received 2 doses of a COVID-19 vaccine as a primary series will receive mRNA-1273.529, mRNA-1273.214, or mRNA-1273 as the 3rd dose, and participants who have previously received a primary series and 1 booster dose will receive mRNA-1273.529, mRNA-1273.214, or mRNA-1273 as the 4th dose (a mixed approach is acceptable). Each part will include Phase A (randomized, blinded) and Phase B (open-label, observational).
This is a 2-part study to evaluate the immunogenicity, safety, and reactogenicity of mRNA-1273.529 and mRNA-1273.214 administered as a booster dose.