The purpose of this study is to evaluate the efficacy and safety of mRNA-1283 and mRNA-1273 (variant formulations) in adults 50 to 64 years of age without high risk factors for severe COVID-19.

Key Inclusion Criteria:

• Healthy and without underlying medical conditions that may put them at higher risk of experiencing severe outcomes of COVID-19, as determined by medical evaluation including medical history, physical examination, laboratory tests, and cardiac monitoring.

Key Exclusion Criteria:

• History of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection within 3 months prior to enrollment.

• Acutely ill or febrile within 72 hours prior to or at the Screening Visit or Day 1.

• Receipt of any of the following:

  • COVID-19 vaccine within 3 months prior to enrollment,
  • Any licensed non COVID-19 vaccine within 28 days before or planned receipt within 28 days after the study intervention, except an influenza vaccine, which may be given 14 days before or after receipt of the study intervention,
  • Corticosteroids at ≥10 milligrams (mg)/day of prednisone or equivalent for >14 days in total within 90 days prior to Day 1 or is anticipating the need for corticosteroids at any time during the study or,
  • Systemic immunosuppressive treatment within 180 days prior to the Screening or is anticipating the need for systemic immunosuppressive treatment at any time during participation in the study.

• Has donated ≥450 mL of blood products within 28 days prior to the Screening Visit or plans to donate blood products within 28 days post study injection.

• Has participated in an investigational 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: Additional inclusion/exclusion criteria may apply, per protocol.