Immune Response to the Seasonal Influenza Vaccine

Background: Vaccines help prevent disease by causing the body to have an immune response. Many parts of this response happen in the blood. This response happens over days and weeks after getting the vaccine. Researchers want to how the blood changes over time in response to vaccines. They want to find out why vaccines work better for some people than for others. This could help make more effective vaccines. Objective: To learn about how the body responds to vaccines. Eligibility: Healthy people ages 18 and older Design: Participants will be screened with a medical history, physical exam, and blood and urine tests. Participants will have 9 visits over 6 months. All visits will include blood tests and a physical exam. Participants will have the first visit 1 week before they get the vaccine. Participants will get the flu vaccine at the second visit. The vaccine will be injected into the muscle of the upper arm with a needle. They will be watched for side effects for 15 minutes. Participants will have the next 2 visits exactly 1 day and 1 week after they get the vaccine. They will have the other 5 visits about 14, 28, 70, 100, and 180 days after they get the vaccine. Participants will take email questionnaires about whether they had any side effects. Participants may have optional extra study visits. These will be no more than once a month for up to 1 year after they get the vaccine. Optionally, they can also repeat the study each year through the 2023 - 2024 flu season