After getting fully vaccinated for HBV why does the antiHBs titer last for so long?
There is such a thing as memory plasma cells, discovered only relatively recently. Here is a review from 2010 discussing them, here is another review from 2012, and here is a pre-publication manuscript of an upcoming review in Leukemia (I apologize if any are behind paywalls, let me know if you can't access any of them).