Examples
Worked examples
- Is an instance
During an outbreak investigation, 555 people meet the case definition for a gastrointestinal illness and 3 die from it; the case fatality rate is (3 / 555) x 100 = 0.5%.
- Is an instance
A hospital tracking a healthcare-associated infection identifies 40 confirmed cases in a quarter, of whom 6 die with the infection recorded as a contributing cause; the case fatality rate for that quarter is (6 / 40) x 100 = 15%.
Counter-examples
Looks similar, but isn't
- Not an instance
A public-health report stating that a disease caused 12 deaths per 100,000 general population in a given year is reporting a mortality rate, not a case fatality rate, because the denominator is the whole population rather than the count of confirmed cases -- a CFR requires that every person in the denominator has already met the case definition.
Editorial commentary
The case fatality rate (CFR), sometimes called the case fatality ratio, measures the severity of a disease among people already known to have it: the share of confirmed cases who die from that disease within a defined period.
Formula
CFR (%) = (number of deaths attributed to the disease ÷ number of confirmed cases of the disease) × 100.
Both the numerator and the denominator are counts of people who have already met the case definition for the disease — the CFR does not draw on the general population at all.
Case fatality rate vs. mortality rate vs. attack rate
- Case fatality rate — denominator is confirmed cases of the disease. Answers “of the people who got this disease, what share died from it?”
- Mortality rate — denominator is the total population (or population-time) at risk, whether or not they became cases. Answers “how many people in the population died of this disease?” and is affected by how common the disease is, not just how deadly it is once contracted.
- Attack rate (fatal outcomes only) — denominator is the population exposed during a bounded outbreak episode, not confirmed cases. See attack rate for the full distinction.
Why CFR estimates during an active outbreak are unstable
A CFR calculated while an outbreak is still unfolding is prone to two biases that pull in opposite directions: case-count lag (mild or asymptomatic cases go undiagnosed, inflating the apparent CFR because the denominator is undercounted) and outcome lag (some currently-confirmed cases have not yet reached their final outcome — recovery or death — deflating the apparent CFR because the numerator is still incomplete). A final, stable CFR generally cannot be reported with confidence until an outbreak has resolved and case ascertainment is complete.
Related CASRAI terms
- Attack rate — the analogous proportion for any defined outcome among the exposed population at risk, not deaths among confirmed cases.
- Person-time — the time-based denominator used for true incidence and mortality rates, as distinct from the case-based denominator used for CFR.
- Outbreak investigation steps — the field-epidemiology sequence within which case counts and deaths are tallied.
- What is epidemiology? — broader context on the discipline and its measures of disease frequency and severity.
Machine-readable encodings
Use in your systems
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