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In epidemiology, a 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 particular period. 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 an outbreak with a high daily increase and long resolution time would be substantially lower than the final CFR.

Terminology

The ''mortality rate''  –  often confused with the CFR  –  is a measure of the relative number of deaths (either in general, or due to a specific cause) within the entire population per unit of time. A CFR, in contrast, is the number of dead among the number of diagnosed cases only. Sometimes the term ''case fatality ratio'' is used interchangeably with ''case fatality rate'', but they are not the same. A case fatality ratio is a comparison between two different case fatality rates, expressed as a ratio. It is used to compare the severity of different diseases or to assess the impact of interventions. From a mathematical point of view, CFRs, which take values between 0 and 1 (or 0% and 100%, ''i.e.'', nothing and unity), are actually a measure of risk  (''case fatality risk'') – that is, they are a proportion of incidence, although they don't reflect a disease's incidence. They are neither rates, incidence rates, nor ratios (none of which are limited to the range 0–1). They do not take into account time from disease onset to death.

Infection fatality rate

Like the case fatality rate, the term infection fatality rate (IFR) also applies to infectious disease outbreaks, but represents the proportion of deaths among all infected individuals, including all asymptomatic and undiagnosed subjects. It is closely related to the CFR, but attempts to additionally account for inapparent infections among healthy people. The IFR differs from the CFR in that it aims to estimate the fatality rate in both sick and healthy infected: the detected disease (cases) and those with an undetected disease (asymptomatic and not tested group). (Individuals who are infected, but show no symptoms, are said to have "unapparent", "silent" or "subclinical" infections and may inadvertently infect others.) By definition, the IFR cannot exceed the CFR, because the former adds asymptomatic cases to its denominator.

Example calculation

If 100 people in a community are diagnosed with the same disease, and 9 of them subsequently die from the effects of the disease, the CFR 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, if the unresolved cases were included as recovered in the earlier analysis. Alternatively, it might later be established that a higher number of people were infected with the pathogen, resulting in an IFR lower than the CFR.

Examples

A half dozen examples will suggest the range of possible CFRs for diseases in the real world: *The CFR for the Spanish (1918) flu was >2.5%, 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 about 15%. *The CFR for yellow fever, even with good treatment, ranges from 20 to 50%. *Bubonic plague, left untreated, will have a CFR of as much as 60%. *Zaïre 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'', nearly always results in death, with a case fatality rate greater than 95%. *Rabies virus, if infecting an unvaccinated individual who does not seek treatment, is also extremely deadly, with a CFR of almost 100%.