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 7months after the planned first dose (in the home, socially, or occupationally) to atleast 1 child ≤5 years of age. Direct exposure is defined as either participant isthe 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 anemancipated minor), or parent(s)/legally acceptable representative (LAR)(s), asapplicable, understand and are willing and physically able to comply withprotocol-mandated follow-up including all study visits and procedures anticipatedduring the 30 month study period.

• Female participants of child-bearing potential: Urine pregnancy test is negative atScreening and negative on the day of the first injection (Day 1). If the participantis sexually active with men, has practiced adequate contraception or has abstainedfrom all activities that could result in pregnancy for at least 28 days prior to thefirst injection (Day 1) and agrees to continue adequate contraception through 3months following the third study injection (Month 9/Day 257).

Extension substudy:

• All consenting participants who were CMV-seronegative at Baseline in themRNA-1647-P301 main study (including those who did seroconvert and did notseroconvert during the main study), received at least one study injection in themain study but did not withdraw study consent, and completed the final study visitin mRNA-1647-P301 main study.

• Consenting participants in mRNA-1647-P301 main study who were CMV-seropositive atBaseline, received all 3 study injections, did not withdraw study consent, andcompleted the final study visit in the main study.

Key Exclusion Criteria:

• History of a diagnosis or condition that, in the judgment of the Investigator, isclinically unstable or may affect participant safety, assessment of safetyendpoints, assessment of immune response, or adherence to study procedures.

• Received or plans to receive any nonstudy vaccine <28 days prior to and after anystudy injection; in addition, the following criteria for COVID-19 and influenzavaccines apply:

• Any COVID-19 primary vaccination series must have been completed a minimum 28 daysprior to receiving any dose of the study injection.

• COVID-19 vaccines (including any booster dose, regardless of manufacturer) must beadministered at least 28 days prior to or after any study injection.

• Influenza vaccines may be administered > 14 days prior to or after any studyinjection.

• Received systemic immunosuppressants or immune-modifying drugs for >14 days in totalwithin 6 months prior to the day of first injection (Day 1) (for corticosteroids, ≥5milligrams (mg)/day of prednisone equivalent) or plans to do so during the course ofthe study. Inhaled, nasal, and topical steroids are allowed. Stable immunomodulatorregimens 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 dayof 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 offirst injection.

• Participated in an interventional clinical study <28 days prior to the day of firstinjection (Day 1) or plans to do so while enrolled in this study.

• Participant has donated ≥450 milliliters (mL) of blood products <28 days prior toScreening.

• Participant is a member of study team or is an immediate family member or householdmember 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 affectparticipant safety, assessment of safety endpoints, assessment of immune response,or adherence to study procedures, including any medical, psychiatric, oroccupational condition that, in the opinion of the Investigator, might poseadditional risk due to participation in the study or could interfere with theinterpretation of study results.