A Study to Investigate the Immunogenicity and Safety of mRNA COVID-19 Variant-containing Vaccine Formulations

Key Inclusion Criteria: * mRNA-1273.167 and mRNA-1273.712: Previously vaccinated adults aged ≥18 years * mRNA-1273.251: Participants ≥65 years of age, ≥12 to \<65 years of age, or ≥6 months to \<12 years of age with at least 1 risk factor for severe outcomes from COVID-19. * mRNA-1273.261: ≥65 years of age at the time of signing the informed consent or ≥18 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. * Able to comply with study procedures based on the assessment of the Investigator. * Female participants of childbearing potential may be enrolled in the study if the participant fulfills all the following criteria: * Has a negative pregnancy test at the Screening Visit and on the day of vaccination prior to vaccine dose being administered on Day 1. * 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. * Has agreed to continue adequate contraception through 28 days following vaccine administration. Key Exclusion Criteria: * History of SARS-CoV-2 infection within 3 months prior to enrollment for mRNA-1273.167 and mRNA-1273.712 or within up to 6 months prior to enrollment for mRNA-1273.251 and mRNA-1273.261. * Is acutely ill or febrile (temperature 38.0°Celsius/100.4°Fahrenheit) less than 72 hours prior to or at the Screening Visit or Day 1. Participants meeting this criterion may be reschedu…