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 14 days in total within 6 months prior to the day of enrollment (for corticosteroids, ≥20 mg/day prednisone equivalent). Topical tacrolimus is allowed if not used within 14 days prior to the day of enrollment. Participants may have visits rescheduled for enrollment if they no longer meet this criterion within the Screening Visit window. Inhaled, nasal, and topical steroids are allowed.
• Intravenous blood products (red cells, platelets, immunoglobulins) within 3 months prior to enrollment
• Has donated ≥450 milliliters (mL) of blood products within 28 days prior to the Screening Visit (Day 0) or plans to donate blood products during the study
• Participated in an interventional clinical study within 28 days prior to the Screening Visit (Day 0) or plans to do so while participating in this study
• Is an immediate family member or has a household contact who is an employee of the research center or otherwise involved with the conduct of the study

For Part 1C-1 and Part 1C-2:

• Pregnant or breastfeeding.
• Is acutely ill or febrile 24 hours prior to or at the Screening Visit (Day 0). Fever is defined as a body temperature ≥ 38.0°C/≥ 100.4°F. Participants who meet this criterion may have visits rescheduled within the relevant study visit windows. Afebrile participants with minor illnesses can be enrolled at the discretion of the investigator.
• Has a medical, psychiatric, or occupational condition that may pose additional risk as a result of participation, or that could interfere with safety assessments or interpretation of results according to the investigator's judgment.
• History of a diagnosis or condition (after enrolment in Part 1A) that, in the judgment of the investigator, may affect study endpoint assessment or compromise participant safety:
• Suspected active hepatitis
• Has a bleeding disorder that is considered a contraindication to IM injection or phlebotomy
• Dermatologic conditions that could affect local solicited AR assessments
• History of anaphylaxis, urticaria, or other significant AR requiring medical intervention after receipt of a vaccine
• Diagnosis of malignancy (excluding nonmelanoma skin cancer)
• Receipt of:
• Any authorized or licensed vaccine within 28 days before the first dose of IP or plans for receipt of any licensed vaccine through 28 days following the last dose of IP or any seasonal influenza vaccine within 14 days before the first dose of IP or plans for receipt of any seasonal influenza vaccine 14 days following the last dose of IP.
• Participated in an interventional clinical study, other than mRNA-1273-P203 study, within 28 days prior to the Screening Visit (Day 0 [for Part 1C-1], BD-Day 0 [for Part 1C-2]) or plans to do so while participating in this study.

Part 1C-2 Heterologous Booster Dose:

• Has a known history of SARS-CoV-2 infection within 2 weeks prior to administration of IP or known close contact with anyone with laboratory-confirmed SARS-CoV-2 infection or COVID 19 within 2 weeks prior to administration of IP.