There’s an ongoing debate about whether SARS-CoV-2 is becoming milder. At Bloomberg Max Nisen says that the preponderance of the evidence suggests it is not. However
If the virus isn’t shifting into a milder form, what else can explain why it doesn’t seem as bad in some places? The fact is, many other things could be at work. The virus may well appear weaker in areas on the other side of a peak because expanded testing and surveillance are catching people earlier in the course of their illness, as opposed to months ago when most were only tested if they got sick enough to brave a packed hospital. Clinicians also know more about how to treat people, and are no longer as overwhelmed.
A solid portion of those most vulnerable due to a weakened immune system or other factors have likely already been infected, especially in hard-hit areas in Italy. Those that haven’t are better protected by public health measures, a lower level of community spread, possible temporary seasonal effects, and at least some degree of acquired immunity in the population.
That last explanation sounds pretty credible to me. Another possibility is that the first cohort of people who contracted the disease were already pretty sick for other reasons. Now even those with serious cases of COVID-19 are stronger and healthier. Also, I suspect that the 1,001st case you treat doesn’t look nearly as bad as the first one did.