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

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.