Common Pitfalls in the Interpretation of COVID-19 Data and Statistics

Paper by Andreas Backhaus: “…In the public debate, one can encounter at least three concepts that measure the deadliness of SARS-CoV-2: the case fatality rate (CFR), the infection fatality rate (IFR) and the mortality rate (MR). Unfortunately, these three concepts are sometimes used interchangeably, which creates confusion as they differ from each other by definition.

In its simplest form, the case fatality rate divides the total number of confirmed deaths by COVID-19 by the total number of confirmed cases of infections with SARS-CoV-2, neglecting adjustments for future deaths among current cases here. However, the number of confirmed cases is believed to severely underestimate the true number of infections. This is due to the asymptomatic process of the infection in many individuals and the lack of testing capacities. Hence, the CFR presumably reflects rather an upper bound to the true lethality of SARS-CoV-2, as its denominator does not take the undetected infections into account.

The infection fatality rate seeks to represent the lethality more accurately by incorporating the number of undetected infections or at least an estimate thereof into its calculation. Consequently, the IFR divides the total number of confirmed deaths by COVID-19 by the total number of infections with SARS-CoV-2. Due to its larger denominator but identical numerator, the IFR is lower than the CFR. The IFR represents a crucial parameter in epidemiological simulation models, such as that presented by Ferguson et al. (2020), as it determines the number of expected fatalities given the simulated spread of the disease among the population.

The methodological challenge regarding the IFR is, of course, to find a credible estimate of the undetected cases of infection. An early estimate of the IFR was provided on the basis of data collected in the course of the SARS-CoV-2 outbreak on the Diamond Princess cruise ship in February 2020. Mizumoto et al. (2020) estimate that 17.9% (95% confidence interval: 15.5-20.2) of the cases were asymptomatic. Russell et al. (2020), after adjusting for age, estimate that the IFR among the Diamond Princess cases is 1.3% (95% confidence interval: 0.38-3.6) when considering all cases, but 6.4% (95% confidence interval: 2.6–13) when considering only cases of patients that are 70 years and older. The serological studies that are currently being conducted in several countries and localities serve to provide more estimates of the true number of infections with SARS-CoV-2 that have occurred over the past few months….(More)”.