Case fatality rate

The case fatality rate (CFR) is the share of people with a given disease or condition who die from it, usually expressed as a percentage. Deaths from the disease among diagnosed cases are divided by the total number of those cases, so if 20 of 1,000 diagnosed patients die of an infection, its CFR is 2 percent. Because the result is a proportion with no time unit, many epidemiologists prefer the name case fatality ratio.

The CFR measures severity, not how common a disease is. A mortality rate divides deaths by a whole population, whereas the CFR divides only by the cases. A rare disease can therefore have a very high CFR, while a widespread disease with a low CFR may still cause many deaths.

The figure depends heavily on which cases are counted. Early in an outbreak, testing tends to find the sickest patients, so mild and symptomless infections are missed and the CFR is overestimated. Deaths also lag behind diagnoses, so a calculation made too soon can underestimate it. The infection fatality ratio, which divides deaths by all infections, including undiagnosed ones found through antibody surveys, is lower. These problems were widely discussed during the COVID-19 pandemic, when crude national CFRs varied enormously.