A Study to Evaluate the Efficacy, Safety, and Immunogenicity of mRNA-1647 Cytomegalovirus (CMV) Vaccine in Healthy Participants 16 to 40 Years of Age

Key Inclusion Criteria: * Participants aged ≥20 years, has or anticipates having direct exposure within 7 months after the planned first dose (in the home, socially, or occupationally) to at least 1 child ≤5 years of age. Direct exposure is defined as either participant is the parent, or participant has close contact (feeding, diaper changes, childcare/supervision) for at least 8 hours per week. * CMV-seronegative Cohort is CMV-seronegative based on CMV testing at Screening. * CMV-seropositive Cohort is CMV-seropositive based on CMV testing at Screening. * Investigator assessment confirms that the participant (including in the case of an emancipated minor), or parent(s)/legally acceptable representative (LAR)(s), as applicable, understand and are willing and physically able to comply with protocol-mandated follow-up including all study visits and procedures anticipated during the 30 month study period. * Female participants of child-bearing potential: Urine pregnancy test is negative at Screening and negative on the day of the first injection (Day 1). If the participant is sexually active with men, has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first injection (Day 1) and agrees to continue adequate contraception through 3 months following the third study injection (Month 9/Day 257). Extension substudy: * All consenting participants who were CMV-seronegative at Baseline in the mRNA-…