In epidemiology, a case fatality rate — sometimes called 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. 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
A mortality rate — often confused with a CFR — is a measure of the number of deaths in a population scaled to the size of that population per unit of time. A CFR, in contrast, is the number of dead among the number of diagnosed cases. Technically, CFRs, which take values between 0 and 1, are actually a measure of risk — that is, they are a proportion of incidence. They are not rates, incidence rates, or ratios. They do not always 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.
Infection fatality rate
The term infection fatality rate also applies to infectious disease outbreaks, and represents the proportion of deaths among all the infected individuals. It 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 and those with an undetected disease. The IFR will always be lower than the CFR as long as all deaths are accurately attributed to either the infected or the non-infected class.
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 at the time of analysis, a later analysis might take into account additional deaths and arrive at a higher estimate of the CFR. Alternatively, it might later be established that a higher number of people were infected with the disease, resulting in a lower CFR.
COVID-19 has had measured CFR's ranging from 1% in Iceland through 4.6% in the U.S. and 5% in China, to 14% in Italy and the UK. The World Health Organisation has estimated the IFR at 0.6%.
Legionnaires' disease has a CFR of ≈15%.
Yellow fever has a CFR of about 3–7.5%.
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, caused by the unicellularNaegleria fowleri, nearly always results in death, with a CFR >99%.
Rabies virus is almost invariably fatal if left untreated, and so has a CFR approaching 100%; however, patients generally respond well to prompt post-exposure prophylaxis.
Prion diseases are progressive and always fatal, regardless of treatment.
