The purpose of this study is to evaluate safety, tolerability and immunogenicity of mRNA-1345 in participants who have been previously vaccinated with either Arexvy or Abrysvo at least 12 months prior to enrollment, are medically stable and aged ≥60 years.

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 Visit 1 due to treatment failure or toxicity.
  • Absence of serious or significant medical events within 30 days of Visit 1.
  • Absence of known, current, and life-limiting diagnoses which, in the opinion of the Investigator, would make completion of the protocol unlikely.

• Participant has received a single dose of Arexvy or Abrysvo at least 12 months prior to Visit 1. Participants must provide proof of RSV vaccination status (including brand and date received) prior to enrollment.

Key Exclusion Criteria:

• Participant has received or plans to receive any vaccine authorized or approved by a local health agency ≤14 days prior to study injection (Day 1) or plans to receive a vaccine authorized or approved by a local health agency within 14 days after the study injection. Nonstudy vaccination(s) should not be delayed.
• Prior participation in research involving receipt of any investigational RSV product (drug/biologic).
• Has received systemic immunoglobulins, long-acting biological therapies that affect immune responses (for example, infliximab) or blood products within 90 days prior to Visit 1 or plans to receive them during the study.

Other protocol-specified inclusion and/or exclusion criteria may apply.