The purpose of this Phase IIb study is to evaluate the efficacy of the RSV vaccine candidate for the prevention of lower respiratory tract disease (LRTD) due to RSV.

The study will enroll approximately 4500 adults aged 60 years and older in a 1:1 ratio to receive a single intra-muscular (IM) administration of either a pre-determined dose of the RSV vaccine candidate or placebo.

Study duration per participant:

6 months.

Treatment Duration:

1 IM injection. Participants will be followed for approximately 6 months post- vaccination.

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. To be considered of non-childbearing potential, a female must be postmenopausal for at least 1 year or surgically sterile.

• Participants with high-risk medical conditions who are determined by medical history, physical examination (if required), and clinical judgment of the investigator to be eligible for inclusion.

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

• Known or suspected congenital or acquired immunodeficiency; or receipt of immunosuppressive therapy, such as anti-cancer chemotherapy or radiation therapy, within the preceding 6 months; or long-term systemic corticosteroid therapy (prednisone or equivalent for more than 2 consecutive weeks within the past 3 months)
• 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 an mRNA vaccine
• History of RSV-associated illness, diagnosed clinically, serologically, or microbiologically in the last 12 months
• Previous history of myocarditis, pericarditis, and/or myopericarditis
• Thrombocytopenia, contraindicating IM injection based on investigator's judgment
• Bleeding disorder or receipt of anticoagulants in the 3 weeks preceding inclusion, contraindicating IM injection
• 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
• Receipt of any vaccine in the 4 weeks preceding any study intervention administration or planned receipt of any 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 with a licensed or investigational vaccine or planned receipt during study participation, of vaccination against RSV with a licensed or investigational vaccine other than the study vaccine
• 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 Note: The above information is not intended to contain all considerations relevant to a patient's 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

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