Study of a Respiratory Syncytial Virus Candidate Encapsulated in a Lipid Nanoparticle Based Formulation in Adults Aged 18 to 50 Years and 60 Years and Older

Inclusion Criteria: * Sentinel Cohort Stage 1: Aged 18 to 50 years on the day of inclusion * Main Cohort Stage 1 and Stage 2: Aged 60 years or older on the day of inclusion Stage 1 and Stage 2: * Sentinel Cohort: 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 OR * Is of childbearing potential and agrees to use an effective contraceptive method or abstinence from at least 4 weeks prior to study intervention administration until at least 12 weeks after study intervention administration. * Main and Booster Cohorts: 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. * Able to attend all scheduled visits and to comply with all study procedures * Informed consent form has been signed and dated Exclusion Criteria: * 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 mRNA COVID-19 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 or bleeding disorder, contraindicating IM injection based on investigator's judgment * Bleeding disorder, or receipt of anticoagulants in the 3 weeks preceding inclusion, contraindicating intramuscular 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 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 with an investigational vaccine * Receipt of immune globulins, blood, or blood-derived products in the past 3 months * Receipt of oral or injectable antibiotic therapy within 72 hours prior to the first blood draw * 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 * Deprived of freedom by an administrative or court order, or in an emergency setting, or hospitalized involuntarily * Self-reported or documented human immunodeficiency virus (HIV) detected by any FDA-approved/validated test, hepatitis B virus surface antigen (HBsAg), hepatits B core antibodies (HBcAb), or hepatitis C virus antibodies (HCV Abs), or positive SARS-CoV-2 RT-PCR or antigen test * Identified as an investigator or employee of the investigator or study center with direct involvement in the proposed study, or identified as an immediate family member (ie, parent, spouse, natural or adopted child) of the investigator or employee with direct involvement in the proposed study The above information is not intended to contain all considerations relevant to a potential participation in a clinical trial.