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Infection Fatality Rate (IFR)

\ɪnˈfɛkʃən\ \fəˈtælɪti\ \reɪt\ \(aɪ-ɛf-ɑr)\

In epidemiology, the Infection Fatality Rate (IFR) of a disease is the percentage of people infected with the disease that are expected to die. 

The IFR, together with R0, is used to model how a disease will affect a given population. The Case Fatality Rate (CFR), the percentage of known cases of a disease that cause the patient to die, is often used to estimate the Infection Fatality Rate (IFR). How close the CFR gets to the IFR depends on how well cases of the disease are identified and recorded. If every person who contracts the disease and every death attributable to the disease is known and recorded, then the CFR will equal the IFR.

The CFR is calculated as the number of people known to have died from the disease divided by the number of known cases of people having the disease. For the CFR to equal the IFR, you need two things:

The numerator of this calculation would need to equal the total number people who have died from the disease. This would require complete accuracy of cause of death diagnosis and records.

The denominator of this calculation would need to equal the total number of people who have contracted this disease. This would need to include all mild and asymptomatic cases and all cases where the patient recovered. 

The biggest challenge in estimating the IFR is calculating this denominator. At the beginning of an outbreak of a disease, the CFR is likely to be much higher than the IFR due to milder cases not being reported or recorded (and perhaps not even known by the affected individual).

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