My understanding, @Keepdistance, is that it is from what they've learned from treating people who have succumbed to the disease.
Initially, it appeared as though it was a respiratory issue, hence the mad scramble to acquire hundreds of ventilators, but then it seems as if, rather than needing ventilators, what sick people needed was oxygen to be given.
The really serious stuff that causes major problems, is very sticky blood, which forms clots and messes up major organs like the kidneys, brain and heart.
So very sick people end up needing dialysis (difficult with clotty blood), are predisposed to strokes, heart attacks etc etc.
So, it's from these observations that the changing views around "risk" are coming.
Disclaimer: I'm no expert, in anything. I just read a lot.!