Immunity DOESN'T mean the virus never ends up in your bloodstream again. After all, how else is the immune system meant to deal with it, if it's not in the blood?
Once somebody becomes infected for the first time or reacquires the virus, there is a window of time before that person becomes infectious and can pass it onto others. With a first time infection, the immune system takes too long to acquire immunity, and so people reach the infectious stage. With immunity and picking the virus up again, the immune system responds much quicker, and is likely to
eliminate it before the person becomes infectious and can pass it onto others.
Herd immunity is about reducing transmission levels. The fewer people who reach the infectious stage, the fewer can become further infected.