What I am going to write now is pure speculation and no kind of daft cloud cuckoo land belief.
I have never had flu and neither have most of my family. My parents never did, nor my sisters. During the Asian flu epidemic of 1957, or thereabouts my sisters and I were at boarding school sleeping in big open dormitories, so many pupils and teachers got flu the school closed. We went home - to an army base - another institutional setting, where again flu was rampant, yet none of us caught the illness. At one point DS's wife and children both had COVID, one of them twice. DS didn't get it, neither has my sister had it
Many years ago I read, in the New Scientist, that about 45% of the population are naturally immune to flu and I have wondered whether the immunity is specifically to flu, or more generally to corona viruses - and flu is caused by a corona virus - in which case we would not get COVID either, which we haven't.
As I said, this is pure speculation on my part, no more.