If a vaccine protects you for most of your life against a particular pathogen, why do you need a new flu shot every year? A. Influenza evolves very quickly and its antigens mutate so that your memory cells from the last vaccine won’t recognize it. B. Influenza is so strong that you need a booster every year to make sure your body can fight it. C. Influenza is a virus, and vaccines only generate memory cells for bacterial infections. D. You don’t need it every year, it is just offered every year for anyone who does not have it yet.