A Phase I Randomized, Observer-Blinded, Dose-Ranging Study in Healthy Subjects 24 to <72 Months of Age

Inclusion Criteria: 1. Healthy males and females, ≥24 months of age and \<72 months of age, without known chronic cardiopulmonary disease including especially persistent or frequently recurrent wheezing. 2. Free of other illnesses that are believed to increase the risks associated with influenza or other respiratory viral infections, including: diabetes mellitus, congenital or acquired blood dyscrasias, renal or hepatic dysfunction, and morbid obesity. 3. Parent(s)/guardian(s) willing and able to give informed consent prior to study enrollment, and assert ability to comply with study requirements. 4. Parent(s)/guardian(s) and/or other designated child care provider must have continuous capacity for telephone communication with the study site. Exclusion Criteria: 1. Serum MN titers against RSV/A or RSV/B \<1:16 (4 log2). 2. Toxicity grade ≥2 for any safety laboratory parameter. 3. Participation in research involving an investigational product (drug/biologic/device) within 45 days before planned date of first vaccination and planned participation at any time during the study. 4. History of a serious reaction to any prior vaccination, including Guillain-Barré Syndrome within six weeks following a previous influenza vaccination. 5. Receipt of any vaccine (other than the influenza vaccine specified in the protocol) in the four weeks preceding the first study vaccination and/or planned receipt of a licensed vaccine (other than the hepatitis A vaccine specified in the protocol) at any time prior to Study Day 56. 6. Receipt of an RSV vaccine at any time. 7. Any known or suspected immunosuppressive condition, acquired or congenital, as determined by medical history and/or physical examination. 8. Chronic administration (defined as more than 14 continuous days) of immunosuppressants or other immune-modifying drugs within 6 months prior to the administration of the study vaccine. An immunosuppressant dose of glucocorticoid will be defined as a systemic dose ≥1 mg/kg of prednisone per day or equivalent. The use of topical glucocorticoids for minor cutaneous symptoms will be permitted, but the use of nasal or inhaled glucocorticoids in exclusionary (because of potential related diagnoses rather than immunosuppression). 9. Administration of immunoglobulins and/or any blood products within the three months preceding the administration of the study vaccine or during the study. 10. Acute disease at the time of enrollment (defined as the presence of a moderate or severe illness with or without fever or an oral temperature \>38.0°C on the planned day of vaccine administration). 11. Known disturbance of coagulation. 12. Any condition that in the opinion of the investigator would pose a health risk to the subject if enrolled or could interfere with evaluation of the vaccine or interpretation of study results (including social, neurologic, or psychiatric conditions in the subject or parent(s)/guardian(s) deemed likely to impair the quality of safety reporting).