The purpose of this study is to evaluate the immunogenicity and safety of the recommended variant-containing formulations of mRNA-1283 COVID-19 vaccine (mNEXSPIKE®) within the current epidemiological environment.

This study uses a master protocol with a generic study design to allow flexibility in evaluating immunogenicity of variant-containing formulations of the mRNA-1283 COVID-19 vaccine. As different variant-containing vaccine formulations will be evaluated, each study intervention (for example, updated mRNA-1283 COVID-19 vaccine) will be described in the subprotocol. For each evaluation of an updated variant vaccine, a new subprotocol will be generated.

Subprotocol 01 is a single arm study to evaluate immunogenicity and safety of mRNA-1283 (COVID-19 vaccine) 2025-2026 formula in individuals aged ≥65 years and ≥12 to <65 years with at least 1 underlying condition that puts them at high risk for severe outcomes from COVID-19.

The study included Parts A and B. Part A enrollment was discontinued due to addition of Part B to evaluate the safety and immunogenicity of variant-containing mRNA-1283 using a hypothesis driven study endpoint with reactogenicity data collection and longer safety follow up in Part B.

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 immune response, or adherence to study procedures.
• Receipt of the COVID-19 vaccine within 6 months prior to enrollment.
• Receipt of 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.
• Receipt of systemic immunosuppressants for >14 days in total, within 180 days prior to Screening Visit (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.
• Receipt of systemic immunoglobulins, long-acting biological therapies that affect immune responses (for example, infliximab) or blood products within 90 days prior to the Screening Visit or plans to receive them during the study.
• History of anaphylaxis or severe hypersensitivity reaction requiring medical intervention after receipt of any mRNA vaccine or therapeutic or any components of an mRNA vaccine or therapeutic.
• Has donated ≥450 milliliters (mL) of blood products within 28 days prior to the Screening Visit or plans to donate blood products within 28 days after the study injection.

Note: Other protocol-specified inclusion and exclusion criteria may apply.