A Study of mRNA-1283 Injection Compared With mRNA-1273 Injection in Participants ≥12 Years of Age to Prevent COVID-19

Key Inclusion Criteria

• Investigator's assessment that the participant understands and is willing and physically able to comply with protocol-mandated follow-up, including all procedures.
• 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 the first injection, and agreement to continue adequate contraception or abstinence through 90 days following the vaccine administration.
• Part 1: Has previously received a primary series of an authorized/approved COVID-19 vaccine. For participants ≥18 years of age, at least 1 booster dose must have also been received. Proof of prior vaccination is required. A heterologous vaccine regimen is acceptable.

Part 2: No prior vaccination is required. For participants who have been previously vaccinated, proof of vaccination is required.

Key Exclusion Criteria

• Has had close contact, as defined by the Centers for Disease Control and Prevention (CDC), with someone who had a SARS-CoV-2 infection in the past 14 days, or COVID-19 in the past 10 days.
• Participant is acutely ill or febrile (temperature ≥38.0 degrees Celsius [°C]/100.4 degrees Fahrenheit [°F]) 72 hours prior to or at the Screening Visit or Day 1.
• Any medical, psychiatric, or occupational condition, including reported history of substance abuse, that, in the opinion of the investigator, might pose additional risk due to participation in the study or could interfere with the interpretation of study results.
• Has received systemic immunosuppressants or immune-modifying drugs for >14 days in total within 181 days prior to Screening (for corticosteroids ≥10 milligrams (mg)/day of prednisone equivalent) or is anticipating the need for immunosuppressive treatment at any time during participation in the study.
• Has received or plans to receive any licensed vaccine ≤60 days prior to the study injection (Day 1) or plans to receive a licensed vaccine within 60 days after the study injection.
• Has received systemic immunoglobulins or blood products within 90 days prior to the Screening Visit or plans to receive during the study.
• Has donated ≥450 milliliters (mL) of blood products within 28 days prior to the Screening Visit or plans to donate blood products during the study.
• Has participated in an interventional clinical study within 28 days prior to the Screening Visit based on the medical history interview or plans to do so while participating in this study.

Note: Other inclusion and exclusion criteria may apply.