Case Fatality Rate

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In epidemiology, a Case Fatality Rate (CaFR) 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 CaFR is conventionally expressed as a percentage and represents a measure of disease severity. CaFRs are most often used for diseases with discrete, limited time courses, such as outbreaks of acute infections. A CaFR can only be considered final when all the cases have been resolved (either died or recovered). The preliminary CaFR, for example, during the course of an outbreak with a high daily increase and long resolution time would be substantially lower than the final CaFR.

Terminology

A Mortality Rate — often confused with a CaFR — 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.[1] A CaFR, in contrast, is the number of dead among the number of diagnosed cases.[2]

Technically, CaFRs, 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 “CaFR” 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.[3][4]

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 CaFRs, expressed as ratio. It also can be used to compare different diseases or to assess the impact of an intervention.[5]

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 CaFR, but attempts to additionally account for all asymptomatic and undiagnosed infections.[6] The IFR differs from the CaFR 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).[7] (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 CaFR as long as all deaths are accurately attributed to either the infected or the non-infected class.


References

  1. For example, a diabetes mortality rate of 5 per 1,000 or 500 per 100,000 characterises the observation of 50 deaths due to diabetes in a population of 10,000 in a given year. (See Harrington, Op. cit..)
  2. "Coronavirus: novel coronavirus (COVID-19) infection"
  3. Entry “Case fatality rate” in Last, John M. (2001), A Dictionary of Epidemiology, 4th edition; Oxford University Press, p. 24.
  4. Hennekens, Charles H. and Julie E. Buring (1987), Epidemiology in Medicine, Little, Brown and Company, p. 63.
  5. "Attack rates and case fatality"
  6. "Infection fatality rate"
  7. "Global Covid-19 Case Fatality Rates"
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