Study of the Safety and Immunogenicity of Reduced Doses of the US Yellow Fever Vaccine

Inclusion Criteria: 1. Males and Females 18 to 50 years of age. 2. In good health, as determined by pertinent medical history, physical examination, vital signs, and clinical safety laboratory evaluations. 3. Female of child bearing potential: Has a negative pregnancy test and is willing to use a reliable form of contraception for the duration of the study after vaccination. 4. Negative human immunodeficiency virus (HIV) antibody screen, seronegative for hepatitis B surface antigen (HBsAg) and hepatitis C antibody (following HIV and hepatitis testing, subjects will be provided with counseling and referral for health care if any test is positive). 5. Ability to comprehend and a willingness to sign an informed consent, which includes the Health Insurance Portability and Accountability Act (HIPAA) Authorization, and a separate consent form for HIV testing. 6. Be willing to comply with all follow-up visits, testing, adverse event (AE) reporting, and completion of diary card. Exclusion Criteria: 1. Receipt of any other investigational vaccine or investigational drug within 28 days prior to or after vaccination with YF-VAX® vaccine. 2. Have had any known flavivirus disease or receipt of any flavivirus vaccine, licensed or investigational at any time; in addition to any yellow fever vaccine, these include; Japanese Encephalitis (JE), St. Louis Encephalitis, Tick Borne Encephalitis (TBE), West Nile, Dengue, Zika virus 3. Anticipates receipt of any other vaccine within 28 days of YF-VAX®. Influenza vaccination will be permitted but not within 14 days of YF-VAX®. 4. Acute or chronic medical conditions, or medications that, in the Principal Investigator's (PI) opinion, would impair the subject's ability to respond to vaccination. 5. Hypersensitivity to any vaccine, eggs or egg products, or allergy to any component of the YF-VAX® (sorbitol, gelatin) or latex. 6. Corticosteroids even ≥20 mg/day of prednisone for ≥ 2 weeks suppresses the immune system. Low-dose corticosteroid topical products and nasal sprays used sporadically (i.e. prn--according to circumstances) are permissible. 7. History of immunosuppression, by any cause--primary or acquired immunodeficiencies, transplantation, malignant neoplasm, lymphoma, leukemia, thymoma, myasthenia gravis, radiation, immunosuppressive drugs, including antimetabolites, tumor necrosis factor (TNF)-alpha inhibitors (etanercept), interleukin-1 (IL-1) blocking agents and other monoclonal antibodies targeting immune cells (e.g., rituximab, alemtuzumab, etc), etc. 8. Receipt of or anticipates receipt of/or donation of blood or blood products for 2 months after receipt of YF-VAX®. (Note: Blood banks require a minimum 2 week interval between the receipt of this FDA licensed vaccine and blood donations; however because of the blood collections in this study, an interval of 2 months is requested). 9. Female: Pregnant (or planning to become pregnant) or breastfeeding for the duration of the study after receipt of YF-VAX®. 10. Clinically significant abnormal laboratory tests (generally 2 times the upper limit of normal or as determined by the PI).