A Study to Evaluate the Immunogenicity and Safety of mRNA-1283 COVID-19 Variant-containing Formulations

Inclusion Criteria: Subprotocol 1: * ≥65 years of age at the time of signing the informed consent or ≥12 to \<65 years of age at the time of signing the informed consent with at least 1 risk factor for severe outcomes from COVID-19. * Investigator's assessment that the participant understands and is willing and physically able to comply with protocol-mandated follow-up, including all procedures. * Participants who are assigned female at birth or could become pregnant: i) Has a negative pregnancy test at the Screening Visit and on the day of vaccination prior to vaccine dose being administered on Day 1. ii) Has practiced adequate contraception or has abstained from all activities that could result in pregnancy for at least 28 days prior to the first dose (Day 1). iii) Has agreed to continue adequate contraception through 28 days following vaccine administration. Exclusion Criteria: Subprotocol 1: * History of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection within 6 months prior to enrollment. * Acutely ill or febrile (temperature ≥38.0°Celsius/≥100.4°Fahrenheit) within 72 hours prior to or at the Screening Visit or Day 1. Participants meeting this criterion may be rescheduled within the Screening window and will retain their initially assigned participant number. * History of a diagnosis or condition that, in the judgment of the Investigator, is clinically unstable or may affect participant safety, assessment of study endpoints, assessment of i…