# Case fatality rate

In epidemiology, a case fatality rate (CFR) — sometimes called case fatality risk or case-fatality ratio — is the proportion of deaths from a certain disease compared to the total number of people diagnosed with the disease for a certain period of time. A CFR is conventionally expressed as a percentage and represents a measure of disease severity.[1] CFRs are most often used for diseases with discrete, limited time courses, such as outbreaks of acute infections. A CFR can only be considered final when all the cases have been resolved (either died or recovered). The preliminary CFR, for example, during the course of an outbreak with a high daily increase and long resolution time would be substantially lower than the final CFR.

A video discussing the case fatality rate and the basic reproduction number in the context of the COVID-19 pandemic.

## Terminology

A mortality rate — often confused with a CFR — is a measure of the number of deaths (in general, or due to a specific cause) in a population scaled to the size of that population per unit of time.[2] A CFR, in contrast, is the number of dead among the number of diagnosed cases.[3]

Technically, CFRs, which take values between 0 and 1 (or 0% and 100%, i.e., nothing and unity), are actually a measure of risk — that is, they are a proportion of incidence. They are not rates, incidence rates, or ratios (none of which are limited to the range 0-1). Hence, even though the terms “case fatality rate” and “CFR” appear often in the scientific literature, if one wishes to be very precise, they are incorrectly used, because they do not always, in every instance, take into account time from disease onset to death.[4][5]

Sometimes the term case fatality ratio is used interchangeably with case fatality rate, but they are not the same. Case fatality ratio is the comparison between two different case fatality rates, expressed as ratio. It also can be used to compare different diseases or to assess the impact of an intervention.[6]

## Infection fatality rate

The term infection fatality rate (IFR) also applies to infectious disease outbreaks, and represents the proportion of deaths among all the infected individuals. It is closely related to the CFR, but attempts to additionally account for all asymptomatic and undiagnosed infections.[7] The IFR differs from the CFR in that it aims to estimate the fatality rate in all those with infection: the detected disease (cases) and those with an undetected disease (asymptomatic and not tested group).[8] (Individuals who are infected, but always remain asymptomatic, are said to have "inapparent" — or silent, or subclinical — infections.) The IFR will always be lower than the CFR as long as all deaths are accurately attributed to either the infected or the non-infected class.

## Example calculation

100 people in a community are diagnosed with the same disease; subsequently 9 of them die from the effects of the disease. The CFR at this point in time, therefore, would be 9%.

If some of the cases have not yet resolved (neither died nor fully recovered) at the time of analysis, a later analysis might take into account additional deaths and arrive at a higher estimate of the CFR.

## Real-world examples

The following examples will suggest the range of possible CFRs for diseases in the real world: