The main purpose of study is to evaluate the safety and immunogenicity of different dose levels of mRNA-1647 versus control in healthy cytomegalovirus (CMV)-seronegative and CMV-seropositive female and male participants 9 to 15 years of age. In addition, mRNA-1647 will be evaluated in female participants 16 to 25 years as a comparator cohort.

The study will be conducted in 2 parts: Part 1 Dose-Ranging and Part 2 Safety Expansion.

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 the time of consent.
• Is in good general health, in the opinion of the Investigator, and is capable of complying with study procedures.
• For the CMV-seronegative cohorts: At the Screening visit, is CMV IgG-negative and CMV immunoglobulin M (IgM)-negative.
• For the CMV-seropositive cohorts: At the Screening visit, is CMV IgG-positive and CMV IgM-negative, CMV IgG-positive and CMV IgM-positive, or CMV IgG-positive and CMV IgM-indeterminate. Participants with an isolated positive or indeterminate result for CMV IgM (that is, CMV IgG-negative and either CMV IgM-positive or CMV IgM-indeterminate) will not be eligible for enrollment but may be rescreened after at least 6 weeks from the initial CMV Screening. Participants with an indeterminate result for CMV IgG, regardless of IgM result, will not be eligible for enrollment but 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 third percentile 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, adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to Day 1, agreement to continue adequate contraception through 3 months following vaccine administration.

Key Exclusion Criteria:

• Has a history of a diagnosis or condition that, in the judgment of Investigator, is clinically unstable or may affect participant safety, assessment of safety endpoints, assessment of immune response, or adherence to study procedures. Clinically unstable is defined as diagnosis or condition requiring significant changes in management or medication within the 2 months prior to Screening and includes ongoing workup of an undiagnosed illness that could lead to a new diagnosis or condition.
• Has received, or plans to receive, any nonstudy vaccine < 28 days prior to or after any study injection.
• Has a screening liver function test (aspartate aminotransferase, alanine aminotransferase, total bilirubin) or a screening creatinine result of Toxicity Grade ≥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 degrees Fahrenheit [°F]) at the Screening Visit.
• Has received systemic immunosuppressants or immune-modifying drugs for > 14 days in total within 6 months prior to the day of enrollment (for corticosteroids, ≥1 milligrams (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 day of the first study injection (Day 1) or plans to do so during the course of the study.
• Reports a history of myocarditis, pericarditis, or myopericarditis.
• Has reported medical history of hepatitis B, hepatitis C, or human immunodeficiency virus (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 day of the first study injection (Day 1).
• Has donated ≥ 450 milliliter (mL) of blood products <28 days prior to the day of the first study injection (Day 1).
• Has participated in an interventional clinical study <28 days prior to the day of the first study injection (Day 1) or plans to do so while enrolled in the study.

Note: Other inclusion and exclusion criteria may apply.

Part 1 of this study will be open-label and blinding is not applicable. Part 2 of the study will be observer-blinded.