@LordCaramac @luckytran This, except that "sterile immunity" is a myth these days due to antivaxxers spreading their garbled version of it ("getting infected without getting infectious", which does not happen in real life).
The actual term is "sterilizing immunity" and it simply means zero breakthrough infections, and is actually the exception, not the rule: it is only possible against a small number of pathogens, who need to have particular evolutionary properties (highly conserved genome, low reproductive rate).