Case fatality rate
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A case fatality rate (CFR, also case fatality risk, or case fatality ratio) is the ratio of deaths from a certain disease to the total number of people diagnosed with this 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. CFR can only be considered final when all the cases have been resolved (either died or recovered). The CFR number during the course of an outbreak with a high daily increase and long resolution time would be substantially lower than the final CFR.
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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. Technically, CFRs, which take values between 0 and 1, are actually a measure of risk (i.e., 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 through the term case fatality rate and CFR often appear in the scientific literature, if one wishes to be precise, the term is incorrectly used, because it does not take into account time from disease onset to death.
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 (either died or recovered) at the time of analysis, this could lead to bias in estimating the CFR.
The following examples will suggest the range of possible CFRs for diseases in the real world:
- The CFR for the Spanish (1918) flu was >2.5%, but about 0.1% for the Asian (1956-58) and Hong Kong (1968-69) flus, and <0.1% for other influenza pandemics.
- Legionnaires' disease has a CFR of ≈15%.
- Yellow fever, even with treatment, has CFRs in the range of 20-50%.
- Bubonic plague has the best prognosis of the three main variants of plague, but if left untreated, has a CFR >60%.
- Ebola virus is among the deadliest viruses, with a CFR as high as 90%.
- Naegleriasis (also known as primary amoebic meningoencephalitis), caused by the unicellular Naegleria fowleri (a.k.a.the brain-eating amoeba), nearly always results in death, and so have a CFR >99%.
- Rabies virus is almost invariably fatal if left untreated, and so has a CFR approaching 100%; however, it generally responds well to prompt, same-day treatment.
- Prion diseases are progressive and always fatal, regardless of treatment.
- Rebecca A. Harrington, Case fatality rate at the Encyclopædia Britannica
- For example, a diabetes mortality rate of 5 per 1,000 or 500 per 100,000 characterizes the observation of 50 deaths due to diabetes in a population of 10,000 population, in a given year.[according to whom?]
- Taubenberger, Jeffery K.; David M. Morens (January 2006). "1918 influenza: the mother of all pandemics". Emerging Infectious Diseases. Coordinating Center for Infectious Diseases, Centers for Disease Control and Prevention. 12 (1): 15–22. doi:10.3201/eid1201.050979. PMC 3291398. PMID 16494711. Archived from the original on 2009-10-01. Retrieved 2009-04-17.
- Li, F C K; B C K Choi; T Sly; A W P Pak (June 2008). "Finding the real case-fatality rate of H5N1 avian influenza". Journal of Epidemiology and Community Health. 62 (6): 555–559. doi:10.1136/jech.2007.064030. ISSN 0143-005X. PMID 18477756. Retrieved 2009-04-29.