If you look at the Italian data, their % positive level never crossed ~30%, and overall averaged <20%. That might indicate that susceptibility for this agent is

If you look at the Italian data, their % positive level never crossed ~30%, and overall averaged <20%. That might indicate that susceptibility for this agent is well below 50%. The more likely explanation is that social distancing was implemented early enough, and tests were given to people who might or might not have had it. There is no reason to believe that we burned through most of the vulnerable people in the population.