I found this research interesting because of personal anecdote:

My family and I have joked that my mom must have some kind of immunity to covid. Before a vaccine was available, she had several direct (and sometimes lengthy) exposures... but never any (noticeable) symptoms or illness. And after being vaccinated with the monovalent rounds, she had several direct exposures to omicron... and still nothing

I myself have worked the front lines as an ICU nurse in the US since the first wave, and have never had covid, that I know of. Never symptomatic and never tested positive (granted, I am much more fastidious in regards to infection prevention). The kicker on my end is that my wife is immunosuppressed, and she also has never gotten covid, even with me working covid units and coming home to her (also why I have been so incredibly anal about not bringing it home).

Could be luck, or anything in-between, but I still wonder if my mom and I are resistant. We both have had many (sometimes severe) respiratory infections when I was growing up, and I've wondered if those exposures to other Corona viruses helped train our immune systems.

Like I said, that's my personal anecdote. We both still get every vaccine for covid-19 that becomes available, because fuck all that. But I've always been curious what future studies of the immune system might reveal. Because we have been wildly lucky, all things considered