One possible reason for what appears to be lower death rates from the CV could be this:-
'One alternative suggestion is that infectious doses of the Covid-19 virus, transmitted from one person to another, may be getting smaller thanks to social distancing. Lower doses would then be easier for our immune systems to tackle, so death rates would drop.'
More details can be found here.
While some scientists believe the virus has become less deadly, others look at the factors that suggest otherwise
www.theguardian.com