If I pick 1k people with mild cases at random, I will find pretty much any strain that currently is in circulation. How do I know which strain is harmless?
I do describe this in my blog post.
Aka this being one mechanism by which you confirm it is only producing mild illness. There's a limited amount you can find out by asking because nobody knows what strain they had.
If it is a rare strain, how will you know if it's safe? Seems like it would have to be a rare strain with thousands of cases and hospitalizations in that population are extremely rare.
Sampling bias and other confounding factors would be a real problem in this search, at least from a statistical point of view, imo.
I also think finding everyone who has been infected with a strain isn't feasible, at best you are sampling from the population.