The purpose of this study is to assess the safety and immunogenicity of a single intramuscular (IM) injection of different doses of an respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) vaccine candidate, in adult participants aged 60 years and older. In addition, the study will evaluate the safety and immunogenicity of a booster vaccination administered 12 months after the primary vaccination in a subset of the study population.

Study duration per participant:

12 months for the Sentinel and Main Cohorts and 24 months overall for the subset of participants enrolled in the Booster Cohort.

Treatment Duration:

Sentinel Cohort: 1 IM injection. Participants will be followed for 12 months post-vaccination.

Main Cohort: 1 IM injection. Participants will be followed for 12 months post-vaccination.

Booster Cohort: 1 IM injection 12 months after the primary vaccination (Main Cohort). Participants will be followed for 12 months after administration of the booster dose.

Inclusion Criteria:

• Aged 60 years or older on the day of inclusion
• A female participant is eligible to participate if she is not pregnant or breastfeeding and is of non-childbearing potential.
• Participants who are determined by medical history, physical examination (if required), and clinical judgment of the investigator to be eligible for inclusion Note: Participants with pre-existing stable disease, defined as disease not requiring significant change in therapy or hospitalization for worsening disease during the 6 weeks before enrollment, can be included.

Exclusion Criteria:

Participants are excluded from the study if any of the following criteria apply:

• Any screening laboratory parameter with laboratory abnormalities deemed clinically significant by the investigator
• Known or suspected congenital or acquired immunodeficiency; or receipt of immunosuppressive therapy. Persons living with stable human immunodeficiency virus (HIV) are not excluded
• Known systemic hypersensitivity to any of the study intervention components (eg, polyethylene glycol, polysorbate); history of a life-threatening reaction to the study interventions used in the study or to a product containing any of the same substances; any allergic reaction (eg, anaphylaxis) after administration of mRNA coronavirus disease 2019 (COVID-19) vaccine
• History of RSV and/or hMPV-associated illness, diagnosed clinically, serologically, or microbiologically in the last 12 months
• Previous history of myocarditis, pericarditis, and/or myopericarditis
• Screening electrocardiogram that is consistent with possible myocarditis, pericarditis, and/or myopericarditis or, in the opinion of the investigator, demonstrates clinically relevant abnormalities that may affect participant safety or study results
• Thrombocytopenia or bleeding disorder, contraindicating intramuscular (IM) injection based on investigator's judgment
• Receipt of anticoagulants in the 3 weeks preceding inclusion
• Chronic illness that, in the opinion of the investigator, is at a stage where it might interfere with study conduct or completion
• Alcohol, prescription drug, or substance abuse that, in the opinion of the investigator, might interfere with the study conduct or completion
• History of acute infection symptoms or a positive severe acute respiratory syndrome coronavirus reverse transcription polymerase chain reaction (SARS-CoV-2 RT-PCR) or antigen test in the 10 days prior to the visit. A prospective participant should not be included in the study until the condition has resolved
• Receipt of any vaccine other than mRNA vaccine in the 4 weeks preceding any study intervention administration or planned receipt of any vaccine other than mRNA vaccine in the 4 weeks following any study intervention administration
• Receipt of any mRNA vaccine in the 60 days preceding any study intervention administration or planned receipt of any mRNA vaccine in the 60 days following any study intervention administration
• Previous vaccination against RSV and/or hMPV (with a licensed or investigational vaccine either as a monovalent vaccine or any combination of the antigens)
• Receipt of immune globulins, blood, or blood-derived products in the past 3 months
• Participation at the time of study enrollment (or in the 4 weeks preceding the first study intervention administration) or planned participation during the present study period in another clinical study investigating a vaccine, drug, medical device, or medical procedure The above information is not intended to contain all considerations relevant to a potential participation in a clinical trial.

Qualified researchers may request access to patient level data and related study documents including the clinical study report, study protocol with any amendments, blank case report form, statistical analysis plan, and dataset specifications. Patient level data will be anonymized and study documents will be redacted to protect the privacy of trial participants. Further details on Sanofi's data sharing criteria, eligible studies, and process for requesting access can be found at: https://vivli.org

Sentinel, Main, and Booster Cohorts: observer-blind; for safety evaluation purposes, the Sponsor will be unblinded

• Investigators, laboratory personnel, and participants will be blinded
• Sponsor study staff and study staff preparing/administering the study interventions will be unblinded