Case fatality rate

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.[2] A CFR, in contrast, is the number of deaths among the number of diagnosed cases only, regardless of time or total population.[3]

From a mathematical point of view, by taking values between 0 and 1 or 0% and 100%, CFRs 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.[4][5]

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.[6]

Because the CFR is not an incidence rate by not measuring frequency, some authors note that a more appropriate term is case fatality proportion.[7]

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 subclinically infected with the pathogen, resulting in an IFR below the CFR.

A CFR may only be calculated from cases that have been resolved through either death or recovery. The preliminary CFR, for example, of a newly occurring disease with a high daily increase and long resolution time would be substantially lower than the final CFR, if unresolved cases were not excluded from the calculation, but added to the denominator only.

${\displaystyle {\text{CFR in }}{\%}={\frac {\text{Number of deaths from disease}}{\text{Number of confirmed cases of disease}}}\times 100}$[8]

Like the case fatality rate, the term infection fatality rate (IFR) also applies to infectious diseases, 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.[9] 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).[10] Individuals who are infected, but show no symptoms, are said to have inapparent, 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.

${\displaystyle {\text{IFR in }}{\%}={\frac {\text{Number of deaths from disease}}{\text{Number of infected individuals}}}\times 100}$[8]

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%,[11] about 0.1% for the Asian (1956-58) and Hong Kong (1968-69) flus,[12] and <0.1% for other influenza pandemics.[11]
• Legionnaires' disease has a CFR of about 15%.[13]^: 665
• The CFR for yellow fever is estimated to be around 5-6% (40-50% for severe cases, which make up around 12% of all cases).[14] [15][16]
• Bubonic plague, left untreated, will have a CFR of as much as 60%.[17]^: 57 With antibiotic treatment, the CFR for septicaemic plague is 45%, pneumonic 29% and bubonic 17%.[18][19]
• Zaïre Ebola virus is among the deadliest viruses with a CFR as high as 90%.[20]
• Naegleriasis (also known as primary amoebic meningoencephalitis), caused by the unicellular Naegleria fowleri, has a case fatality rate greater than 95%.
• Rabies virus has a CFR of 99 to 100% in unvaccinated individuals.[21]

• List of human disease case fatality rates
• Mortality rate – Deaths per 1000 individuals per year
• Pandemic severity index – Proposed measure of the severity of influenza

• Definitions of case fatality for coronary events in the WHO MONICA Project
• Swine flu: what do CFR, virulence and mortality rate mean?