Flu Clinical Trials

The goal of this clinical trial is to compare immune responses to different influenza vaccine formulations in older adults and to understand how immune aging and repeated annual influenza vaccination affect these responses. The main questions it aims to answer are: * Do standard dose, MF59 adjuvanted, and high dose influenza vaccines produce different immune responses in adults aged 65 years or older? * How does immune aging affect responses to different influenza vaccine formulations? * How do immune responses change with repeated annual influenza vaccination, and do these changes differ among vaccine formulations? Researchers will compare older adults who receive a standard dose, MF59 adjuvanted, or high dose influenza vaccine. Younger adults aged 19-49 years who receive a standard dose influenza vaccine will also be included as a reference group to help evaluate age related differences in immune responses. Participants will: * Receive an influenza vaccine. * Provide blood samples before and after vaccination to measure antibody and other immune responses. * Some participants will provide additional blood or nasal samples for more detailed evaluation of immune responses.