as far as i can tell; the vaccine preps your body to have to deal with it. so, your still gonna be 'ready' for it. but, if everyone else is carrying it, then your body is going to have to deal with it, again, and again, and again. And each time your body has to deal with it, there is a chance that the virus will evolve and overcome you. Or that the virus will evolve in others and then overcome you.