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 percent in most of the US  and at least 90 percent in most of Europe. These are much larger and faster changes than reported in empirical studies, such as the 18 percent decrease in morality found for the New York City hospital system from March to August 2020 1%DIFDELCMD < ]%%% . 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 .
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 %DIFDELCMD < ]%%% than those previously reported and surprisingly do not correlate well with our model parameters (such as the contact rate or 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 .
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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
Our compartment model shows that the COVID-19
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seems to have
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states
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of Europe. These
European countries. These drops
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changes than reported in empirical studies, such as the 18 percent decrease in morality found for the New York City hospital system from March to August 2020
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. Our reported decreases
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have strong correlations to other
correlate well with our
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contact rate) or other standard
the contact rate or testing rate) or
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the
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Almost all the decreases
Surprisingly, these changes mostly
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, which unexpectedly corresponds to the time when many state and national lockdowns were released resulting in
-- at a time when lockdown relaxations in many states and countries caused
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Several plausible
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examined
discussed
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treatment
treatments
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potentially changing
age
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are overwhelmingly convincing given the currently available evidence
alone support such large decreases