Characterization of Human Memory Immune Responses to Prior Yellow Fever Vaccination

Inclusion Criteria: * Able to understand and give informed consent * Age 18-45 years (Group 1) or 55 years and above (Group 2) at the time of yellow fever vaccination * Documentation (international certificate of vaccination (yellow card) or medical record) indicating receipt of yellow fever vaccine. Exclusion Criteria: * Recipient of any vaccines within 30 days before the study visit (not applicable to older adults) * History of a progressive and severe chronic medical condition resulting in impaired immunity (such as diabetes, kidney or liver dysfunction) * Required use of immunosuppressive medications * Reporting HIV, Hepatitis B (surface antigen positive) or Hepatitis C infections (antibody positive) on the medical/health history form * Recipient of a blood product or immune globulin product within 42 days of study visit * Reporting pregnancy