Unpacking the Drop in COVID-19 Case Fatality Rates: A Study of National and Florida Line-Level Data

Abstract

Since the COVID-19 pandemic began, the United States's case fatality rate (CFR) has plummeted. Using national and Florida data, we unpack the drop in CFR between April and December 2020, accounting for such confounders as expanded testing, age distribution shift, and detection-to-death lags. Guided by the insight that treatment improvements in this period should correspond to decreases in hospitalization fatality rate (HFR), and using a block-bootstrapping procedure to quantify uncertainty, we find that although treatment improvements do not follow the same trajectory in Florida and nationally (with Florida undergoing a comparatively severe second peak), by December, significant improvements are observed both in Florida and nationally (at least 17% and 55% respectively). These estimates paint a more realistic picture of improvements than the drop in aggregate CFR (70.8%-91.1%). We publish a website where users can apply our analyses to selected demographics, regions, and dates of interest.

Figure 1:

From left to right: confirmed cases, deaths, and case fatality rate, calculated using 7-day trailing averages based on national reporting data available via USAFacts data pulled from the Carnegie Mellon Delphi project’s COVIDcast API. Data outside the April 1st to December 1st time range considered in this study is grayed out.

Figure 2:

COVID-19 positive test rates (right) and tests (left and middle) for Florida and the United States, calculated using 7-day trailing averages and pulled from the COVID Tracking Project. Positive test rate is calculated by dividing new positives by total new tests on each day. Data outside the April 1st to December 1st time range considered in this study is grayed out.

Figure 3:

Age-stratified cases, (eventual) deaths, and (eventual) hospitalizations in Florida and in the U.S., by the date of first positive test result (Florida) and date of report to the CDC (U.S.). Note that the x axis is not the date of death or date of hospitalization.

Figure 4:

Aggregate case fatality rate (left) and hospitalization fatality rate (right) in Florida and in the U.S., by the date of first positive test result (Florida) or date of report to the CDC (U.S.).

Figure 5:

Age distributions among Florida and national cases, (eventual) hospitalizations, and (eventual) deaths, by the date of first positive test result (Florida) and date of report to the CDC (U.S.), respectively.