Epidemiology of disease – concepts used in describing the disease effects on a population - case fatality

Definition

Case fatality rate is the proportion of individuals diagnosed with a specific disease who die from it within a defined period - a measure of disease severity/lethality, distinct from the overall mortality rate (which reflects death rate in the whole population, not just those with the disease)
A key measure for comparing disease severity across different conditions or across different time periods/treatment eras for the same condition, and frequently reported (and sometimes misinterpreted) during infectious disease outbreaks

Case fatality rate is calculated only among confirmed/diagnosed cases (deaths divided by total diagnosed cases), meaning it is directly affected by how completely cases are ascertained - under-ascertainment of mild/asymptomatic cases inflates the calculated case fatality rate, since the denominator (total cases) is artificially reduced while the numerator (deaths, typically well-ascertained) remains relatively complete, explaining why early outbreak case fatality rate estimates are often revised downward as testing/case ascertainment improves

Should be interpreted alongside an understanding of case ascertainment completeness for the specific disease/outbreak being described, given the systematic bias toward overestimation when many mild cases go undiagnosed
Distinct from and should not be confused with overall population mortality rate, which uses the whole population (not just diagnosed cases) as the denominator

Comparing case fatality rates across treatment eras can help assess (though not definitively prove, given other confounding factors) improvement in disease management effectiveness over time
Early outbreak case fatality rate estimates should be interpreted cautiously, anticipating likely downward revision as case ascertainment improves with better testing availability
Case ascertainment bias inflating early outbreak case fatality rate estimates (a frequently tested, practically important point), case fatality rate vs overall mortality rate distinction, denominator dependence on diagnosed cases only
Case fatality rate estimates early in a disease outbreak are typically overestimates, since testing/diagnostic capacity is usually most limited early on, meaning many mild or asymptomatic cases go undetected and are excluded from the denominator while deaths (the numerator) are more completely captured - this systematic bias explains why reported case fatality rates commonly fall as an outbreak progresses and testing capacity improves

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