when you get a vaccination against pretty much anything, the immune system gets into a sort of alert state for a couple of days. Normally it would take a few days to kick the specific immune system to do anything, but when it has been alerted like this, it can hit the virus pretty much instantaneously and so it has much easier time beating it. I've at least read reports that this could have happened to some people. However I'm not really sure that anybody understands this at all.
(I only wonder how you can enjoy any food without having a sense of taste! Or maybe your friend was just enjoying the fact that he could eat it without feeling as if he was going to breathe fire? )
By the way (and very fortunately!) the long-term effects seem to go away with time, it just takes several (6-12?) months, apparently.