This was very true for the H1N1 epidemic but it's not clear something like this is happening with Covid. If anything, the opposite might be happening where prior infection with a related virus makes the current infection much worse.
I wonder how valid this bit still is:
> the discrepancy between the severity of cases observed in the Hubei province of China and those occurring elsewhere in the world
I think at this point we’ve already seen very severe cases in many other countries, no? I don’t have proper sources though, just thinking of misc news articles I’ve read.