A Long-Term Extension Study to Evaluate the Immunogenicity and Safety of Cytomegalovirus (CMV) mRNA-1647 Vaccine

The study aims to explore the long-term potential differences in immunogenicity and key safety parameters following mRNA-1647 administration, participants who completed study mRNA-1647-P202, who are CMV-seronegative and who did not seroconvert due to primary CMV infection during the study will be enrolled. Study mRNA-1647-P202 participants who were CMV-seropositive at baseline, randomized to receive mRNA-1647 injection, and completed their final study visit will also be enrolled. No study treatment will be administered during the primary extension phase of this study. Eligible participants enrolling in the primary extension phase who completed Study mRNA-1647-P202 may have received mRNA-1647 (either low dose, medium dose, or high dose) or placebo in CMV-seronegative participants and mRNA-1647 (either low dose, medium dose, or high dose) in CMV-seropositive participants. In the optional BP, consenting and eligible participants will receive a single BD of the mRNA-1647 vaccine (medium dose). In the Observational Group of the optional BP, consenting and eligible participants will not receive study treatment.