Studying Cell Immune Responses to a Live Flu Vaccine in Healthy Adults

Background: \- One form of the influenza vaccine is a nasal spray. It uses a live but weakened flu virus. Researchers want to better under how the live vaccine interacts with the body s immune system. They will test the nasal spray flu vaccine (called FluMist) against a saline (salt water) nasal spray. They will then look at blood and nasal cell samples to see how the vaccine affects these cells immune response. Objectives: \- To look at immune changes in nasal and blood cells in people who receive live flu vaccine. Eligibility: \- Healthy volunteers between 18 and 49 years of age. Design: * Participants will have five outpatient visits for this study. Each visit will last up to 2 hours. * At the first visit, participants will have a physical exam and medical history. They will give blood and urine samples. Nasal cell samples will also be collected. * A week later, participants will have either the nasal spray flu vaccine or a saline spray. They will know which spray they will receive. Blood samples will be collected. * Two days after the vaccination, they will have another physical exam. Blood and nasal cell samples will be collected. * At the final two visits (1 week and 1 month after the vaccination), more blood samples will be collected. * Those who had the saline spray will be able to have the actual vaccine spray at the last study visit. * The ratio of participants who receive vaccine to those who receive saline will be 4:1.