Case-fatality ratio

The case-fatality ratio (CFR) is the proportion of confirmed cases of a disease that end in death, expressed as a percentage. CFR measures the severity of an illness in those who become recognized cases — it does not measure how widely the pathogen spreads, and it is materially affected by which cases are detected and counted.

Reported CFRs span an enormous range: measles in industrialized settings is below 0.1%; seasonal influenza is roughly 0.1%; SARS-CoV-1 was around 10%; Zaire ebolavirus has ranged from 44% to 90%; rabies once symptoms develop is essentially 100%. Within a single outbreak, CFR typically declines as surveillance improves and milder cases are detected; the late-outbreak CFR is almost always lower than the early-outbreak CFR for the same event.

CFR is sometimes confused with the infection-fatality ratio (IFR), which is the proportion of ALL infections — symptomatic or not, detected or not — that result in death. IFR is always lower than CFR because the denominator is larger. IFR requires seroprevalence surveys or modeling to estimate; CFR only requires case-counting and death-counting.