CFR, IFR, and You: What is the true COVID-19 death rate?

Approximately 6 months off from the official start of the COVID-19 outbreak in China (although the actual start of the outbreak is debated) and over 3 months from the initial nationwide shutdown in the United States, there is much uncertainty as to the true mortality rate of the disease. It is first important to define terms, as there are different types of “mortality” or “fatality” rates.

Defining Case Fatality Rate vs. Infection Fatality Rate

As shown in Figure 1, the reported number of COVID-19 cases does not necessarily represent the true number of infected individuals. The confirmed number of cases (positive tests for active infection) is smaller than the number of probable infected (sick patients believed to be infected based on symptoms and likely exposure but who never received a diagnostic test), which is smaller than the actual number of total infected individuals.

Figure 1. Relationships between measured number of COVID-19 cases via different metrics and number of deaths (not necessarily to scale)

A case fatality rate(CFR) is the proportion of deaths from a disease compared to the number of people diagnosed.

An infection fatality rate (IFR) is the proportion of deaths among all infected individuals, in other words the true fatality rate. While related to the CFR, an IFR attempts to estimate the mortality rate including non-diagnosed cases (e.g. not tested, asymptomatic). An IFR should be lower than the CFR, since the denominator would be expected to be larger.

 

Visually, these can be represented as shown in Figure 2 below, based on the relative differences in confirmed vs. total cases as shown in Figure 1. Note that deaths can be scored as either confirmed deaths or probable deaths. As testing becomes more widespread, the difference between “confirmed” and “total” infected shrinks, as does the difference between CFR and IFR.

Figure 2. Visual representation of Case Infection Rate (CFR) and Infection Fatality Rate (IFR)