Our compartment model shows that the COVID-19 case mortality rate  decreased by at least 80 percent in most of the US states and at least 90 percent in most European countries. These drops are much larger and faster than those previously reported and surprisingly do not correlate well with our model parameters (such as the contact rateor testing rate) or state/national metrics such as the population density, GDP, and median age. Surprisingly, these changes mostly occurred between mid-April and mid-June -- at a time when lockdown relaxations in many states and countries caused surges of new cases. Plausible causes for this drop are discussed , such as improvements in treatments , face mask wearing, a new virus strain, and age demographics of infected patients, but none alone support such large decreases .
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 .
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