You'd have to sample many thousands of people who either have no symptoms or got sick. If the strain only exists in people with no symptoms then there's a good

You'd have to sample many thousands of people who either have no symptoms or got sick. If the strain only exists in people with no symptoms then there's a good chance it's not deadly or at least less deadly than the other strains. Of course you might find the best strain you can find exists in 100% of asymptomatics and 20% of the dead. That would indicate your strain has some non-zero death rate.