Comment by autoexec

If it's been a while you lose the active antibodies, but your immune system still knows whats up and can generate them when exposed. If you're lucky they do it fast enough that the virus doesn't get time to gain a foothold and you never experience a symptom. More likely though you get sick, but your body has a head start fighting off the infection so you don't get as sick. That's why the vaccines help people from severe illness even after a few months.

The bad news is that there are strains out there now that are different enough that even our trained immune systems won't recognize them. That's why it's good to get a booster when updated vaccines become available.