A Study to Evaluate the Safety and Immune Response of mRNA-1345, a Vaccine Targeting Respiratory Syncytial Virus (RSV), When Co-administered With a Fluzone HD, in Adults ≥65 Years of Age

Key Inclusion Criteria: Participants may have one or more chronic medical diagnoses, but should be medically stable as assessed by: Absence of changes in medical therapy within 60 days of Day 1 due to treatment failure or toxicity, Absence of serious or significant medical events within 30 days of Day 1, and Absence of known, current, and life-limiting diagnoses which, in the opinion of the Investigator, would make completion of the protocol unlikely. A participant assigned female at birth is eligible to participate if they are postmenopausal or not a person of childbearing potential. Key Exclusion Criteria: Close contact with someone with laboratory-confirmed influenza and/or RSV infection or with someone who has been treated with antiviral therapies for influenza (for example, Tamiflu®) within the past 5 days prior to Day 1. Reported history of congenital or acquired immunodeficiency, immunosuppressive condition or immune-mediated disease, asplenia, or recurrent severe infections. Participant has tested positive for influenza or RSV by local health authority-approved testing methods ≤6 months prior to Day 1. Participant has received or plans to receive any vaccine authorized or approved by a local health agency ≤28 days prior to study injections (Day 1 and Day 22) or plans to receive a vaccine authorized or approved by a local health agency within 28 days after study injections. Participant has received a seasonal influenza vaccine or any other investigational influenza vaccine ≤6 months prior to Day 1. Participant has received any RSV vaccine (authorized/approved by local health agency or investigational) prior to Day 1. Note: Other protocol-defined inclusion and/or exclusion criteria may apply.