  By fitting a compartment ODE model for Covid-19 propagation to cumulative
case and death data for US states and European countries, we find that the case
mortality rate seems to have decreased by at least 80% in most of the US and at
least 90% in most of Europe. These are much larger and faster changes than
reported in empirical studies, such as the 18% decrease in mortality found for
the New York City hospital system from March to August 2020 (Horwitz et al,
Trends in Covid-19 risk-adjusted mortality rates, J. Hosp. Med. 2020). Our
reported decreases surprisingly do not have strong correlations to other model
parameters (such as contact rate) or other standard state/national metrics such
as population density, GDP, and median age. Almost all the decreases occurred
between mid-April and mid-June, which unexpectedly corresponds to the time when
many state and national lockdowns were released resulting in surges of new
cases. Several plausible causes for this drop are examined, such as
improvements in treatment, face mask wearing, a new virus strain, and
potentially changing demographics of infected patients, but none are
overwhelmingly convincing given the currently available evidence.
