Sorry, that was an oblique response; to accurately answer your question, there are different levels of immunity. A lot of the public's misunderstanding of vaccines is centered around the idea that immunity conferred by vaccination is absolute; it isn't. A vaccine doesn't make one immune, it primes the immune system to detect and fight infection -nothing more, nothing less. In some cases, that priming is so effective that the immune system clobbers a pathogen just as soon as it gets a foot in the door. In other cases, a pathogen is able to slip in under the radar, set up shop and be doing business before the immune system knows what's happening...and then the immune system's SWAT team shows up and goes scorched earth. In other cases still, a pathogen is able to enter the body and set up shop, the immune system sends in the SWAT team but neither side is able to achieve a decisive outcome and a balance is reached wherein the pathogen remains in the body but is killed off at the same rate as it reproduces.