I don’t know this as a scientific fact, but have read that when your immune system meets the flu for the first time, your body makes an antibody for that strain.
Then, every other time in your life you are exposed, your body makes the same antibody, no matter what strain is encountered. That’s why when a strain similar to your first exposure comes around, you don’t get it or only a mild case. When one that’s similar comes around, you are much more susceptible.
The 1957 and 1968 flu outbreaks were terrible. I think everybody in school in 1968 caught that one.
oops - ‘when NOT similar.
I was born in ‘57, so was 11 in ‘68; I may have gotten the flu that year... I remember being sick as a dog :0 .