In epidemiology, a case fatality rate (CFR) — sometimes called case fatality risk — 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. 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 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. So CFR is mortality among diagnosed cases. 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 ratios — none of which are limited to the range of 0 to 1 — nor are they rates or incidence densities. 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.
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.
The term infection fatality rate (IFR) also applies to infectious disease outbreaks, and is closely related to the CFR, but attempts to additionally account for all asymptomatic and undiagnosed infections. 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). (Individuals who are infected, but always remain asymptomatic, are said to have "inapparent" — or silent, or subclinical, or occult — infections.) The IFR will always be lower than the CFR.
Assume 9 deaths among 100 people in a community all diagnosed with the same disease. This means that among the 100 people formally diagnosed with the disease, 9 died and 91 recovered. The CFR, therefore, would be 9%. If some of the cases have not yet resolved (neither died nor recovered) at the time of analysis, this could lead to bias, undercounting deaths and underestimating the CFR.
The following examples will suggest the range of possible CFRs for diseases in the real world: