Vaccines help protect us from diseases, but some work longer than others. The yellow fever vaccine gives protection for many years with just one shot, while the influenza vaccine needs to be taken more often. This study will help us understand why vaccines last for different times by giving either the yellow fever or influenza vaccine to participants. The study will look at how the body responds to the vaccines by collecting samples from blood, lymph nodes (where immune cells are), and bone marrow (where cells that make antibodies live).

• The study is conducted at the Hope Clinic of Emory University and involves multiple visits.
• Participants must be aged 18-50 and cannot receive other vaccines close to study participation.
• Women must use birth control and test negative for pregnancy to join the study.

Being part of the study will help scientists create vaccines that last longer. Participants will receive information about the study and their eligibility before joining. Risks include possible reactions to the vaccine and sample collection procedures, which will be done by experts.