File this one under "biology is complicated and strange." I found a paper claiming that people with blood type O had 2/3 of the risk of contracting covid-19 in China. I was skeptical, but apparently SARS is similar. The antibodies to A and B antigens found in the bloodstream of people with type O reduce binding of SARS to its receptor.
https://academic.oup.com/glycob/article/18/12/1085/1988773
