It's not limitless, but scientific testing has shown it can handle much more than you are imagining.<p><i>I'm not sure how the immunity conferred by the vaccine differs from that conferred by the live virus. It is probably just as good, but what if it isn't?</i><p>It doesn't differ at all. All vaccines are made of live-but-weakened or dead versions of viruses. Your immune system doesn't realize that, and reacts as though it's being attacked. The "hard" part for it is concocting antibodies for the virus it's discovered. And that, naturally, is the beneficial effect of the vaccine, because the immune system doesn't easily forget how to make that kind of antibody. Vaccines work because the immune system reacts the same way to a weak or dead virus that it does to a live, strong one.