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

Inclusion Criteria: Primary Extension Phase: * Male and female participants who were CMV-seronegative at the Study mRNA-1647-P202 Screening visit, completed the final Study mRNA-1647-P202 visit, and remained CMV-seronegative at Screening for Study mRNA-1647-P202-EXT. * Male and female participants who were CMV-seropositive at the Study mRNA-1647-P202 Screening visit, were randomized to receive mRNA-1647 injection (and not placebo), and completed the final visit in Study mRNA-1647-P202. * Understands and agrees to comply with the trial procedures and provides written informed consent. * According to the assessment of the Investigator, is in good general health and is capable of complying with trial procedures. Optional Booster Phase: For BD Recipients: \- CMV-seronegative and CMV-seropositive participants who received mRNA-1647 medium dose injection (and not placebo or other dose levels) in Study mRNA-1647-P202, including participants who enrolled in the Primary Extension Phase of Study mRNA-1647-P202-EXT and participants who were eligible but did not previously enroll in the Primary Extension Phase of Study mRNA-1647-P202-EXT are eligible to receive a BD in the optional BP. For Observational Group: \- CMV-seronegative and CMV-seropositive participants who received mRNA-1647 low dose or high dose injection (and not placebo or mRNA-1647 medium dose injection) in Study mRNA-1647-P202, including participants who enrolled in the Primary Extension Phase of Study mRNA-1647-P202-EXT and participants who were eligible but did not previously enroll in the Primary Extension Phase of Study mRNA-1647-P202-EXT are eligible to enroll in the observational group. Exclusion Criteria: Primary Extension Phase: * Receipt of any CMV vaccine other than mRNA-1647. * Diagnosis or condition that, in the judgment of the Investigator, may affect participant safety, assessment of safety endpoints, assessment of immune response, or adherence to trial procedures, including any medical, psychiatric, or occupational condition that, in the opinion of the Investigator, might pose additional risk due to participation in the study or could interfere with the interpretation of study results. Optional Booster Phase: For BD Recipients: \- Participants from the placebo, mRNA-1647 low dose, or mRNA-1647 medium dose groups in Study mRNA-1647-P202. For Observational Group: \- Participants from the placebo or mRNA-1647 medium dose groups in study mRNA-1647-P202. Note: Other protocol-defined inclusion/exclusion criteria apply