COVID-19: Why Has the Mortality Rate Declined?

This article explains how the mortality rate of an illness such as Coronavirus Disease 2019 (COVID-19) is calculated as well as how the definition of what is a "case" has changed from the earliest days of the pandemic to now. Many factors were not known about The Sudden Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) which causes COVID-19 at the beginning of the pandemic because it is a novel human pathogen. One key factor that was not known in the earliest days of the pandemic was that many patients are either asymptomatic or have symptoms so mild that they may not seek medical attention and hence these patients would not be identified as a "case" if that term is defined as being sufficiently symptomatic to be seeking medical attention. Cases in the earliest days of the pandemic were defined as based on having symptoms (eg, fever, cough, respiratory distress) after ruling out other possible causes. Cases now are defined by tests confirming that the person is shedding the SARS-CoV-2 (ie, a laboratory vs. a symptomatic diagnosis). The mortality rate of this virus dropped as a function of this change. On the basis of the results of an unintended, naturalistic experiment on an expeditionary cruise in March of 2020, there was more than a 5-fold drop in the calculated mortality rate due to this definitional change in what constituted a case. This column explains this issue and discusses its implications for effectively dealing with the SARS-CoV-2 (or COVID-19) pandemic.