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 partici…