This study is testing how long vaccines protect us from getting sick. Some vaccines, like the **yellow fever vaccine (YF-17D)**, can protect you for many years with just one shot. Others, like the **quadrivalent influenza vaccine (QIV)**, don't last as long and need to be given every year. This study will give people either the yellow fever vaccine or the flu vaccine and then take samples to see how their immune system (the body's defense against sickness) responds.

Participants will visit the clinic several times for different tests. These include taking small samples from blood, lymph nodes (glands that help fight infections), and bone marrow (where blood cells are made). This will help researchers understand why some vaccines last longer than others.

• Study Length: Involves several visits, including screening, vaccination, and follow-ups.
• Eligibility: Ages 18-50, must not have certain allergies or medical conditions.
• Risks: Procedures like sample collection may cause discomfort.

This research could help make better vaccines that protect us for a long time, keeping us healthy from diseases.