A Study to Evaluate the Safety and Reactogenicity of DPX-RSV(A), a Respiratory Syncytial Virus Vaccine

Inclusion Criteria: * Age 50-64 years, inclusive. * Good general health status, as determined by history and physical examination no greater than 30 days prior to administration of the test article. * Participants who, in the opinion of the investigator, can and will comply with the requirements of the protocol (e.g. completion of Diary Cards, return for follow-up visits). * Written informed consent obtained from the participant. * If female of child-bearing potential and heterosexually active, has practiced adequate contraception for 30 days prior to injection, has a negative pregnancy test on the day of injection, and has agreed to continue adequate contraception until 180 days after injection. (Please refer to the glossary for the definition of child-bearing potential and adequate contraception). Exclusion Criteria: * Use of any investigational or non-registered product (drug or vaccine) other than the study product within 28 days preceding the dose of study product, or planned use during the study period. * Concurrently participating in another clinical study, at any time during the study period, in which the participant has been or will be exposed to an investigational or a non-investigational vaccine/product (pharmaceutical product or device). * Planned administration/ administration of a vaccine/product not foreseen by the study protocol within the period starting 28 days before injection of a study vaccine and ending 84 days after. * Administration of immunoglobulins and/or any blood products within the 3 months preceding the dose of study product or planned administration during the study period. * Any confirmed or suspected immunosuppressive or immunodeficient condition, based on medical history and physical examination. (Laboratory testing for HIV, Hepatitis C and Hepatitis B will be performed during the screening visit). * Chronic administration (defined as more than 14 days in total) of immunosuppressants or other immune-modifying drug within 6 months prior to the product dose (for corticosteroids, this will mean prednisone ≥ 20 mg/day, or equivalent). Inhaled and topical steroids are allowed. * Family history of congenital or hereditary immunodeficiency. * History of or current autoimmune disease. * History of hypersensitivity to any test article constituent or products used during the course of study procedures. * Known or suspected hypersensitivity to any ingredient in the formulation or component of the container. * Pregnant or lactating female. * Female planning to become pregnant or planning to discontinue contraceptive precautions within 180 days of study vaccine receipt. * Any hematological (hemoglobin level, white blood cell \[WBC\], and platelet count) and biochemical (alanine aminotransferase \[ALT\], aspartate aminotransferase \[AST\], blood urea nitrogen \[BUN\] and creatinine) abnormality as per local laboratory normal values considered clinically significant by the investigator. * Transient mild laboratory abnormalities may be rescreened and the participant will be deemed eligible if the laboratory repeat test is normal as per local laboratory normal values and investigator assessment. * Any acute or chronic, clinically significant disease, as determined by physical examination or laboratory screening tests. * Malignancies within previous 5 years (excluding non-melanic skin cancer) and lymphoproliferative disorders. * Current alcoholism and/or drug abuse. * Acute disease and/or fever at the time of Screening ≥ 38°C 1. Fever is defined as temperature ≥ 38° /100.4°F by any route; the preferred route for recording temperature in this study will be oral. 2. Participants with a minor illness (such as mild diarrhea, mild upper respiratory infection) without fever may be enrolled at the discretion of the investigator. 3. Participants with acute disease and/ or fever at the time of Screening may be re-screened at a later date. * Planned move to a location that will prohibit participating in the trial until study end. * Any other condition that the investigator judges may interfere with study procedures (e.g. drawing blood) or findings (e.g. immune response).