The case fatality rate (CFR) is the proportion of diagnosed cases of a disease that result in death over a specified period. It provides a measure of disease severity among identified cases and is typically expressed as a percentage.
Calculation and interpretation
The CFR is calculated by dividing the number of deaths from a disease by the number of confirmed cases of the disease and multiplying by 100. For example, if a disease causes 50 deaths among 1,000 confirmed cases, the CFR is 5%. Unlike mortality rate, which relates deaths to the entire population, the CFR focuses on outcomes among those known to be affected. Accurate estimation requires reliable case detection and death reporting; delays between diagnosis and death can cause apparent CFR to change over time. Age distribution, comorbidities, access to healthcare, and treatment availability all influence the CFR. During outbreaks of emerging diseases, initial CFR estimates may be biased upward because milder or asymptomatic cases are undercounted. As more cases are detected, the CFR often declines. The CFR is not appropriate for chronic diseases with long survival times because deaths and cases may occur years apart.
Examples and limitations
Epidemiologists use CFRs to compare the lethality of different diseases and to monitor changes over time. For instance, the CFR of untreated bubonic plague historically exceeded 60%, whereas the CFR of seasonal influenza is typically below 0.1%. During the 2014‑2016 West African Ebola outbreak, CFRs varied by region from 40% to 70%. Early in the COVID‑19 pandemic, reported CFRs differed dramatically between countries due to testing capacity and demographic differences; as surveillance improved and treatments emerged, estimates converged to lower values. CFRs for diseases like rabies and variant Creutzfeldt‑Jakob disease approach 100% once clinical symptoms appear. Because CFR depends on detection and reporting practices, it should be interpreted alongside other metrics such as infection fatality rate, which considers undiagnosed infections, and hospitalization rate. CFR does not capture long‑term morbidity or quality‑of‑life impacts.
The case fatality rate offers a snapshot of disease lethality among recognized cases, but it can vary widely with time, place, and population. Careful interpretation is necessary when comparing CFRs across outbreaks or informing public health responses.
Related Terms: Mortality rate, Infection fatality rate, Morbidity, Epidemiology, Outbreak