A Study of mRNA-1647 Cytomegalovirus Vaccine in Healthy Participants 9 to 15 Years of Age and Participants 16 to 25 Years of Age

Key Inclusion Criteria:

• Is a female or male 9 to 15 years of age or is a female 16 to 25 years of age at thetime of consent.

• Is in good general health, in the opinion of the Investigator, and is capable ofcomplying with study procedures.

• For the CMV-seronegative cohorts: At the Screening visit, is CMV IgG-negative andCMV immunoglobulin M (IgM)-negative.

• For the CMV-seropositive cohorts: At the Screening visit, is CMV IgG-positive andCMV IgM-negative, CMV IgG-positive and CMV IgM-positive, or CMV IgG-positive and CMVIgM-indeterminate. Participants with an isolated positive or indeterminate resultfor CMV IgM (that is, CMV IgG-negative and either CMV IgM-positive or CMVIgM-indeterminate) will not be eligible for enrollment but may be rescreened afterat least 6 weeks from the initial CMV Screening. Participants with an indeterminateresult for CMV IgG, regardless of IgM result, will not be eligible for enrollmentbut may be rescreened after at least 6 weeks from the initial CMV screening.

• If 9 to 15 years of age, has a body mass index (BMI) at or above the thirdpercentile according to World Health Organization (WHO) Child Growth Standards. If 16 to 25 years of age: has a BMI of 15 to 35 kilograms (kg)/square meter (m^2).

• For female participants of childbearing potential: negative pregnancy test, adequatecontraception or has abstained from all activities that could result in pregnancyfor at least 28 days prior to Day 1, agreement to continue adequate contraceptionthrough 3 months following vaccine administration.

Key Exclusion Criteria:

• Has a history of a diagnosis or condition that, in the judgment of Investigator, isclinically unstable or may affect participant safety, assessment of safetyendpoints, assessment of immune response, or adherence to study procedures.Clinically unstable is defined as diagnosis or condition requiring significantchanges in management or medication within the 2 months prior to Screening andincludes ongoing workup of an undiagnosed illness that could lead to a new diagnosisor condition.

• Has received, or plans to receive, any nonstudy vaccine < 28 days prior to or afterany study injection.

• Has a screening liver function test (aspartate aminotransferase, alanineaminotransferase, total bilirubin) or a screening creatinine result of ToxicityGrade ≥1.

• Has a Screening hematology or coagulation result of Toxicity Grade ≥1.

• Is acutely ill or febrile (body temperature ≥38.0 degrees Celsius [°C]/100.4 degreesFahrenheit [°F]) at the Screening Visit.

• Has received systemic immunosuppressants or immune-modifying drugs for > 14 days intotal within 6 months prior to the day of enrollment (for corticosteroids, ≥1milligrams (mg)/kg/day or ≥10 mg/day prednisone equivalent).

• Has received an antiviral with activity against CMV (ganciclovir, valganciclovir,foscarnet, cidofovir, letermovir, acyclovir, valacyclovir) <2 weeks prior to the dayof the first study injection (Day 1) or plans to do so during the course of thestudy.

• Reports a history of myocarditis, pericarditis, or myopericarditis.

• Has reported medical history of hepatitis B, hepatitis C, or human immunodeficiencyvirus (HIV); or a positive screening test for hepatitis B surface antigen (HBsAg),hepatitis C antibodies, or HIV 1 or 2 antibodies.

• Has previously received an investigational CMV vaccine.

• Has received systemic immunoglobulins or blood products <3 months prior to the dayof the first study injection (Day 1).

• Has donated ≥ 450 milliliter (mL) of blood products <28 days prior to the day of thefirst study injection (Day 1).

• Has participated in an interventional clinical study <28 days prior to the day ofthe first study injection (Day 1) or plans to do so while enrolled in the study.

Note: Other inclusion and exclusion criteria may apply.