Alzheimer International based in London estimates 1/3 of world wide covid deaths have been people with Alzheimer and could be as high as 45% if undiagnosed individuals and other early stage dementia were included.
It's known that covid hits Alzheimer patients and others with brain disorders harder. There are doctors investigating the theory, that in the same manner covid attaches to the lining of the lungs, it attaches or somehow disables the part of the brain regulating breathing and other functions, perhaps by clogging micro blood vessels in the brain. Since there is a genetic component to Alzheimer, it would stand to reason that your idea that the severity of covid symptoms has a genetic component to it also has merit.