Disease CFR Case Fatality Rate

Understanding the Case Fatality Rate (CFR)

The case fatality rate tells you what share of confirmed cases of a disease end in death. The formula is CFR = deaths / confirmed_cases × 100%. Both the numerator and the denominator count only cases that were confirmed in a lab or diagnosed clinically and then reported to surveillance systems.

There's a catch: CFR almost always overstates how lethal an infection really is. Asymptomatic and mild cases that never get tested simply don't show up in the denominator. During COVID-19, the reported CFR ran anywhere from 1% to 5% depending on the country, how much testing was being done, the age of the population and how late deaths were reported. The actual infection fatality rate (IFR) was far lower than any of those numbers.

Applications

During outbreaks the CDC, WHO and ECDC lean on CFR as their main lethality figure, mostly because surveillance data lets you work it out fast. It feeds into triage protocols, hospital surge planning, who gets vaccinated first and how risk is communicated to the public. For some historical perspective: Ebola CFR ≈50%, SARS ≈10%, MERS ≈35%, seasonal influenza <0.1%, and measles 1–3% in unvaccinated populations.

FAQ

Why does CFR vary so much between countries? It comes down to how much testing happens, the age makeup of the population, the capacity of the health system and how each place defines a case. Where testing is scarce, only the severe cases get caught, which pushes the CFR up.

What is the difference between CFR and mortality rate? The denominator is what separates them. CFR divides by confirmed cases; mortality rate divides by the whole population. So CFR answers “if I get diagnosed, what is my risk of dying,” while mortality rate measures the burden across an entire population.

Should I use CFR or IFR? Reach for CFR when you're doing clinical and surveillance reporting or responding in the early days of an outbreak. Turn to IFR, once seroprevalence studies make it available, when you need the true biological lethality and you're planning for the long haul.