A Study to Evaluate Safety, Reactogenicity, and Immunogenicity of mRNA-1283 and mRNA-1273 Vaccines in Healthy Adults Between 18 Years and 55 Years of Age to Prevent COVID-19

Key Inclusion Criteria: * Understands and agrees to comply with the study procedures and provides written informed consent. * According to the assessment of the investigator, is in good general health and can comply with study procedures. * Body mass index (BMI) of 18 kilograms/square meter (kg/m\^2) to 35 kg/m\^2 (inclusive) at the Screening Visit (Day 0). * 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, agreement to continue adequate contraception or abstinence through 3 months following the second injection, and not currently breastfeeding. Key Exclusion Criteria: * Known history of SARS-CoV-2 infection or known exposure to someone with SARS-CoV-2 infection or COVID-19 in the past 30 days. * Positive serology results for SARS-CoV-2 at the Screening Visit. A negative serological test for SARS-CoV-2, performed on a blood sample obtained at the Screening Visit, is required before a participant can be dosed. * Travel outside of the United States in the 28 days prior to the Screening Visit (Day 0). * Prior administration of an investigational, authorized, or licensed CoV (for example, SARS-CoV-2, SARS-CoV, or Middle East Respiratory Syndrome \[MERS\]-CoV) vaccine, based on medical history interview. * Current treatment with investigational agents for prophylaxis against COVID-19. * Recent (within the last 12 months) use of a dermal filler. * 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. * Has received systemic immunosuppressants or immune-modifying drugs for \>14 days in total within 6 months 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 28 days prior to the first injection (Day 1) or plans to receive a licensed vaccine within 28 days before or after any study injection, with the exception of licensed influenza vaccines, which may be received more than 14 days before the first study injection or more than 14 days after the second study injection. * Receipt of systemic immunoglobulins or blood products within 3 months prior to the Screening Visit (Day 0) or plans for receipt during the study. * Current use of any inhaled substance (for example, tobacco or cannabis smoke, nicotine vapors). * History of chronic smoking (1 cigarette a day) within 1 year of the Screening Visit. * Resides in a nursing home. * 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. * 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.