Epidemiology · Reference
What is the case fatality rate?
The case fatality rate is the proportion of people diagnosed with a particular disease who die from it within a given period. It measures the severity of a disease among recognised cases, and is distinct from the infection fatality rate and from population mortality rates.
What the CFR measures
The case fatality rate captures the lethality of a disease among people known to have it: of those diagnosed, what proportion die from the condition. It is calculated as the number of deaths from the disease divided by the number of diagnosed cases over the same period, typically given as a percentage. Despite the conventional name "rate", it is really a proportion or ratio, because the numerator is a subset of the denominator and there is no person-time involved. A higher CFR indicates a more severe disease among diagnosed cases.
CFR, IFR and mortality rate
Three measures are easily confused. The case fatality rate uses diagnosed cases as its denominator. The infection fatality rate (IFR) uses all infected people, including undiagnosed and asymptomatic infections, so the IFR is usually lower than the CFR because its denominator is larger.
The mortality rate is different again: its denominator is the whole population, not just cases, so it measures deaths in the population rather than among the diseased. Mixing these up leads to large errors in interpreting how dangerous a disease is.
Why the CFR can be hard to estimate
During an ongoing outbreak the CFR is difficult to pin down. If many cases are still ill, dividing current deaths by current cases can underestimate the eventual CFR, because some current cases may die later. Conversely, if mild or asymptomatic cases go undetected, the denominator is too small and the CFR is overestimated. Differences in testing, case definitions and which cases come to attention mean reported CFRs vary between places and over time. Careful epidemiological methods, such as accounting for the time between diagnosis and death, are used to produce more reliable estimates.
Interpreting the CFR responsibly
Because the CFR depends heavily on who counts as a "case", comparisons must hold the case definition and ascertainment constant to be meaningful. As a severity measure it complements frequency measures such as incidence: a disease can have a high CFR but be rare, or a low CFR but affect many people. This page defines the measure and its pitfalls in general, historical terms; it does not provide clinical or prognostic information about any specific condition or individual.
Key facts
At a glance
- Definition: Diagnosed cases who die from the disease
- Formula: Deaths from disease ÷ diagnosed cases (× 100)
- Type: A proportion/ratio, not a true rate
- CFR vs IFR: CFR uses cases; IFR uses all infections (usually lower)
- CFR vs MR: Mortality rate uses the whole population as denominator
Common questions
FAQ
What is the difference between case fatality rate and infection fatality rate?+
The case fatality rate divides deaths by diagnosed cases, while the infection fatality rate divides deaths by all infected people, including undetected and asymptomatic infections. Because the infection fatality rate has a larger denominator, it is usually lower than the case fatality rate for the same disease.
Is the case fatality rate the same as the mortality rate?+
No. The case fatality rate uses diagnosed cases as its denominator and measures lethality among the diseased, whereas the mortality rate uses the whole population as its denominator and measures deaths across everyone, whether or not they had the disease.
Why does the case fatality rate change during an outbreak?+
During an outbreak the CFR can be unstable because some current cases may die later, biasing early estimates downward, while undetected mild cases can bias it upward by shrinking the denominator. Differences in testing and case definitions also cause reported CFRs to vary between settings.
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