Immune Responses to H5N1 Vaccine With and Without AS03 Adjuvant

Background: \- Adjuvants are substances included in vaccines that stimulate the immune system and increase the body's response to the vaccine. The AS03 adjuvant is used in seasonal flu vaccines in Europe. However, it has not been fully tested in the United States. Researchers want to test the immune responses of people who receive a bird flu vaccine with or without AS03 to better understand how the adjuvant works. This information may help develop better flu vaccines. Objectives: \- To compare the healthy immune system responses to bird flu vaccine with or without the AS03 adjuvant. Eligibility: \- Healthy volunteers between 18 and 45 years of age. Design: * Participants will be screened with a physical exam and medical history. * Participants will be assigned to one of two groups. Each group will have an initial vaccine, followed by a booster vaccine 21 days later. Both vaccinations will use the same type of vaccine. One group will have the vaccine with AS03; the other group will have the vaccine without it. * All participants will have two 36-hour inpatient stays to receive the vaccine. Each stay will involve the vaccination, followed by regular and frequent blood draws. Participants will be monitored for any signs or symptoms that may be caused by the vaccine. * Additional blood samples will be collected 7, 28, 42, and 100 days after the initial vaccination.