A Study to Evaluate the Safety, Reactogenicity, and Effectiveness of mRNA-1273 Vaccine in Adolescents 12 to <18 Years Old to Prevent COVID-19

Inclusion Criteria: For Part 1A, Part 2 and Part 3: * Participants 12 to \<18 years of age at the time of consent (Screening Visit, Day 0) who, in the opinion of the Investigator, are in good general health based on review of medical history and screening physical examination. * Investigator assessment that the participant, in the case of an emancipated minor, or parent(s)/legally acceptable representative(s) (LAR\[s\]) understand and is willing and physically able to comply with protocol-mandated follow up, including all procedures and provides written informed consent/assent. * Body mass index (BMI) at or above the third percentile according to World Health Organization (WHO) Child Growth Standards at the Screening Visit (Day 0) * Female participants of nonchildbearing potential may be enrolled in the study. Nonchildbearing potential is defined as premenarche or surgically sterile (history of bilateral tubal ligation, bilateral oophorectomy, hysterectomy). * Female participants of childbearing potential may be enrolled in the study if the participant has a negative pregnancy test at Screening (Day 0), on the day of the first injection (Day 1), on the day of the second injection (Day 29 in Parts 1A and Part 2, and Day 181 in Part 3); 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 has agreed to continue adequate contraception or abstinence through 3 month…