TB and I sort of joke about this because my grandson raises pigs for show.
Still, the 1918 variant immunity is a bit of a stretch ;)
The article that was referenced in post #19 does give some indication of a limited immunity to some people born before 1957.... and it explains the reasoning... (and it sounds plausible). But, that’s different than the immunity by those who did go through the 1918 flu pandemic...