To demonstrate non-inferiority of the immune response of mRNA-1273.529 compared to mRNA-1273 booster administered as a 4th dose against the B.1.1.529 strain at Day 29: 1. Geometric mean titer (GMT) of mRNA 1273.529 and mRNA-1273 against both the B.1.1.529 and prototype strain at Day 29 after study vaccine administration 2. Ratio of GMTmRNA-1273.529/GMTmRNA-1273 against the B.1.1.529 strain at Day 29 after study vaccine administration 3. Geometric mean fold rise (GMFR) of mRNA-1273.529 against the B.1.1.529 strain at Day 29 after study vaccine administration To evaluate the safety and reactogenicity of mRNA-1273.529 and mRNA-1273 administered as a booster dose : 4. Solicited local and systemic reactogenicity ARs during a 7-day follow up period after vaccination 5. Unsolicited adverse events (AEs) during the 28-day follow-up period after vaccination 6. SAEs; medically attended AEs (MAAEs); AEs leading to withdrawal; and AEs of special interest (AESIs) from Day 1 to end of study

To demonstrate non-inferiority of the immune response of mRNA-1273.529 compared to mRNA-1273 booster administered as a 4th dose against the B.1.1.529 strain at Day 29: 1. Geometric mean titer (GMT) of mRNA 1273.529 and mRNA-1273 against both the B.1.1.529 and prototype strain at Day 29 after study vaccine administration 2. Ratio of GMTmRNA-1273.529/GMTmRNA-1273 against the B.1.1.529 strain at Day 29 after study vaccine administration 3. Geometric mean fold rise (GMFR) of mRNA-1273.529 against the B.1.1.529 strain at Day 29 after study vaccine administration To evaluate the safety and reactogenicity of mRNA-1273.529 and mRNA-1273 administered as a booster dose : 4. Solicited local and systemic reactogenicity ARs during a 7-day follow up period after vaccination 5. Unsolicited adverse events (AEs) during the 28-day follow-up period after vaccination 6. SAEs; medically attended AEs (MAAEs); AEs leading to withdrawal; and AEs of special interest (AESIs) from Day 1 to end of study