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 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 contr…