Safety and Immunogenicity Study to Evaluate Single- or Two-Dose Regimens Of RSV F Vaccine With and Without Aluminum Phosphate or Matrix-M1™ Adjuvants In Clinically-Stable Older Adults

Inclusion Criteria: 1. Males and females 60 through 80 years of age who are ambulatory and live in the community or in an assisted-living facility that provides minimal assistance, such that the subject is primarily responsible for self-care and activities of daily living. Subjects may have one or more chronic medical diagnoses, but should be clinically stable as assessed by: * Absence of changes in medical therapy within one month due to treatment failure or toxicity (dose adjustments of ongoing therapies for optimal effect, or replacements within a class of drugs due to convenience or cost, will be deemed acceptable), * Absence of medical events qualifying as SAEs within one month of the planned vaccination on Day 0, and * Absence of known, current, and life-limiting diagnoses which, in the opinion of the investigator, render survival to completion of the protocol unlikely. 2. Willing and able (on both a physical and cognitive basis) to give informed consent prior to study enrollment. To complete the consent process, all qualifying subjects will correctly answer at least 4 out of 5 questions of the informed consent form (ICF) comprehension assessment in no more than 2 attempts. 3. Able to comply with study requirements. As the protocol procedures involve telephone contacts for safety ascertainment, eligible subjects must have a reliable access to a telephone. Exclusion Criteria: 1. Received any prior RSV vaccine. 2. Participation in research involving any additional investigational product (drug / biologic / device) within 45 days before planned date of first vaccination. 3. History of a serious reaction to any prior vaccination or a history of Guillain-Barré syndrome (GBS) within 6 weeks of any prior influenza immunization. 4. Receipt of inactivated influenza vaccine within 14 days prior to the Day 0 dose of test article or any other vaccine within the 4 weeks prior to the Day 0 dose of test article. 5. Any known or suspected immunosuppressive condition, acquired or congenital, as determined by history and/or physical examination. 6. 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 ≥ 10 mg of prednisone per day or equivalent. The use of topical, inhaled, and nasal glucocorticoids will be permitted. 7. Administration of immunoglobulins and/or any blood products within the 3 months preceding the administration of the study vaccine or during the study. 8. 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). 9. Known disturbance of coagulation. Potential subjects receiving aspirin, clopidogrel, prasugrel, dipyridamole, dabigatran, apixaban, rivaroxaban, or warfarin under good control for cardiovascular prophylaxis or prophylaxis of thromboembolic disease or stroke in the setting of atrial fibrillation will NOT be excluded. 10. Suspicion or recent history (within one year of planned vaccination) of alcohol or other substance abuse. 11. 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 neurologic, cognitive, or psychiatric conditions deemed likely to impair the quality of study compliance or safety reporting).