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-1647-P301 main study (including those who did seroconvert and did not seroconvert during the main study), received at least one study injection in the main study but did not withdraw study consent, and completed the final study visit in mRNA-1647-P301 main study. * Consenting participants in mRNA-1647-P301 main study who were CMV-seropositive at Baseline, received all 3 study injections, did not withdraw study consent, and completed the final study visit in the main study. Key Exclusion Criteria: * History of a diagnosis or condition that, in the judgment of the Investigator, is clinically unstable or may affect participant safety, assessment of safety endpoints, assessment of immune response, or adherence to study procedures. * Received or plans to receive any nonstudy vaccine \<28 days prior to and after any study injection; in addition, the following criteria for COVID-19 and influenza vaccines apply: * Any COVID-19 primary vaccination series must have been completed a minimum 28 days prior to receiving any dose of the study injection. * COVID-19 vaccines (including any booster dose, regardless of manufacturer) must be administered at least 28 days prior to or after any study injection. * Influenza vaccines may be administered \> 14 days prior to or after any study injection. * Received systemic immunosuppressants or immune-modifying drugs for \>14 days in total within 6 months prior to the day of first injection (Day 1) (for corticosteroids, ≥5 milligrams (mg)/day of prednisone equivalent) or plans to do so during the course of the study. Inhaled, nasal, and topical steroids are allowed. Stable immunomodulator regimens used for managing environmental allergies are allowed. * Receipt of an antiviral with activity against CMV (ganciclovir, valganciclovir, foscarnet, cidofovir, letermovir, acyclovir, valacyclovir) \<2 weeks prior to the day of first injection or plans to do so during the course of the study. * Previous receipt of an investigational CMV vaccine. * Receipt of systemic immunoglobulins or blood products \<3 months prior to the day of first injection. * Participated in an interventional clinical study \<28 days prior to the day of first injection (Day 1) or plans to do so while enrolled in this study. * Participant has donated ≥450 milliliters (mL) of blood products \<28 days prior to Screening. * Participant is a member of study team or is an immediate family member or household member of study personnel. Extension substudy: * 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 study 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.