A PHASE 2B PLACEBO-CONTROLLED, RANDOMIZED STUDY OF A RESPIRATORY SYNCYTIAL VIRUS (RSV) VACCINE IN PREGNANT WOMEN

Inclusion Criteria - Maternal participants: * Healthy women 18 to 49 years of age between 24 and 36 weeks of gestation on the day of planned vaccination, with an uncomplicated pregnancy, who are at no known increased risk for complications, and whose fetus has no significant abnormalities observed on ultrasound. * Willing and able to comply with scheduled visits, treatment plan, laboratory tests, and other study procedures. * Receiving prenatal standard of care. * Had an ultrasound performed at \>=18 weeks of pregnancy. * Had a negative urinalysis for protein and glucose at the screening visit. Trace protein in the urine is acceptable if the blood pressure is also normal. * Determined by medical history, physical examination, screening laboratory assessment, and clinical judgment to be appropriate for inclusion in the study. * Documented negative human immunodeficiency virus antibody, hepatitis B virus surface antigen, hepatitis C virus antibody, and syphilis tests at the screening visit. * Body mass index of \</=40 kg/m2 at the time of the screening visit. * Capable of giving signed informed consent, which includes compliance with the requirements and restrictions listed in the informed consent document and in this protocol. * Expected to be available for the duration of the study and willing to give informed consent for her infant to participate in the study. Inclusion Criteria - Infant Participants: * Evidence of a signed and dated ICD signed by the parent(s). * Parent(s) willing and able to comply with scheduled visits, treatment plan, laboratory tests, and other study procedures. Exclusion Criteria - Maternal Participants: * Bleeding diathesis or condition associated with prolonged bleeding that would, in the opinion of the investigator, contraindicate intramuscular injection. * History of severe adverse reaction associated with a vaccine and/or severe allergic reaction to any component of the investigational product or any related vaccine. * History of latex allergy. * History of any severe allergic reaction. * Participants with known or suspected immunodeficiency. * Current pregnancy resulting from in vitro fertilization or other assisted reproductive technology. * A prior history of or known current pregnancy complications or abnormalities that will increase the risk associated with the participant's participation in and completion of the study. * Major illness of the mother or conditions of the fetus that, in the investigator's judgment, will substantially increase the risk associated with the participant's participation in, and completion of, the study or could preclude the evaluation of the participant's response. * Participant with a history of autoimmune disease or an active autoimmune disease requiring therapeutic intervention including but not limited to systemic or cutaneous lupus erythematosus, autoimmune arthritis/rheumatoid arthritis, Guillain-Barré syndrome, multiple sclerosis, Sjögren's syndrome, idiopathic thrombocytopenia purpura, glomerulonephritis, autoimmune thyroiditis, giant cell arteritis (temporal arteritis), psoriasis, and insulin-dependent diabetes mellitus (type 1). * Other acute or chronic medical or psychiatric condition including recent (within the past year) or active suicidal ideation or behavior or laboratory abnormality that may increase the risk associated with study participation or investigational product administration or may interfere with the interpretation of study results and, in the judgment of the investigator, would make the participant inappropriate for entry into this study. * Participation in other studies involving investigational drug(s) within 28 days prior to study entry and/or during study participation. * Participants who receive treatment with immunosuppressive therapy including cytotoxic agents or systemic corticosteroids (such as for cancer or an autoimmune disease), or planned receipt of such treatment or agents during study participation. If systemic corticosteroids have been administered short term (\<14 days) for treatment of an acute illness, participants should not be enrolled into the study until corticosteroid therapy has been discontinued for at least 30 days before investigational product administration. Inhaled/nebulized, intra articular, intrabursal, or topical (skin or eyes) corticosteroids are permitted. * Current alcohol abuse or illicit drug use. * Receipt of blood or plasma products or immunoglobulin, from 60 days before investigational product administration, or planned receipt through delivery, with 1 exception, Rho(D) immune globulin (eg, RhoGAM), which can be given at any time. * Previous vaccination with any licensed or investigational RSV vaccine or planned receipt during study participation. * Laboratory test results at the screening visit outside the normal reference value for pregnant women according to their trimester in pregnancy. * Participants who are breastfeeding at the time of the screening visit. Exclusion Criteria - Infant Participants: • Infant who is a direct descendant (eg, child or grandchild) of the study personnel.