This Phase 2 study aims to explore the long-term potential differences in immunogenicity and key safety parameters following mRNA-1647 administration in CMV-seronegative and CMV-seropositive participants.

Study mRNA-1647-P202 participants 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 an mRNA-1647 injection, and completed their final study visit will also be enrolled.

No study treatment will be administered during this extension study. Eligible participants enrolling in the extension study 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.