This study is looking at the safety and immune response to a high-dose flu vaccine in adults aged 50-64. The study will last about 180 days (around 6 months). Immunogenicity means how well your body builds protection after a vaccine. To join the study, you must be aged 50-64, and if you are a woman, you must not be pregnant or must agree to use birth control during the study. You also can't join if you have certain medical conditions like an immune system problem, a history of Guillain-Barré Syndrome (a rare nerve disorder), or if you've recently had another vaccine.

• The study will last about 6 months with several visits needed.
• You should not have had any other vaccines 4 weeks before starting and during the study.
• If you have health issues that might interfere, you might not be eligible.

If you decide to join, make sure you can attend all visits and follow the study rules. This will help doctors learn more about flu vaccines and how they work in older adults.