A Study to Investigate Safety, Reactogenicity, and Immunogenicity of mRNA-1365 (Profile 2) in Older Healthy Adults

Key Inclusion Criteria: * In good general health or with stable chronic medical conditions, as determined by medical evaluation including medical history and physical examination. * Body mass index within the range 18.5 to 35 kg/m2 (inclusive). Key Exclusion Criteria: * 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. * Abnormal screening clinical laboratory values for total white blood cell count, hemoglobin, hematocrit, alanine aminotransferase, aspartate aminotransferase, creatinine, alkaline phosphatase, or total bilirubin \>Grade 1, or platelets ≥Grade 1. * History of a diagnosis or condition that, in the judgment of the Investigator, is clinically unstable, potentially affects participant safety, the assessment of study endpoints, the assessment of immune response, or adherence to study procedures. * History of congenital or acquired immunodeficiency, immunosuppressive condition, asplenia, or recurrent severe infectious disease. * Participants with immune-mediated (autoimmune) conditions are generally excluded. * Received a licensed RSV vaccine or participated in a study in which vaccines against RSV and/or hMPV were administered to participants alone, or as part of a combination vaccine, at any time in the past. Note: Other inclusion and exclusion criteria may apply.