CFR and IFR explained (with a special guest appearance by Typhoid Mary)

“How dangerous is this disease; how likely is a person to die?” is not a simple question.

One way to answer this question is to calculate the case fatality rate (CFR). This is a measurement calculated as the number of deaths from the disease divided by the total number of people who are diagnosed with the disease in a set time period.

Simple, yes? Of course, diagnosing a disease may be easier said than done. We need accurate methods of diagnosis, and we need to know who to test. Which brings us to a name you may have heard before: Typhoid Mary.

In the 1800s, it was assumed that everyone infected with a disease would show obvious symptoms. There was no understanding of asymptomatic carriers, and there were no tests or even understanding of testing for natural immunity developed by exposure to a disease.

From 1900 to 1907, a cook named Mary Mallon worked for many families in the New York City area. Mallon would leave one family as its members developed typhoid fever and gain employment with another family that would soon succumb to typhoid fever as well. Researcher George Soper investigated and published his results in the June 15, 1907 issue of the Journal of the American Medical Association, revealing the existence of silent carriers such as “Typhoid Mary.” Today, we can use tests to detect antibodies and reveal these asymptomatic cases.

Salmonella Typhi, the bacteria that causes typhoid fever. Symptoms of infection can range from mild to fatal, with some carriers being asymptomatic.

Infection fatality rate (IFR) increases the denominator by recognizing these asymptomatic or undiagnosed carriers, and lowers the fatality rate. However, both measurements are also affected by the time from the onset of the infection to the time of death, and may underestimate the fatality rate if patients in intensive care linger for a longer time.

Both CFR and IFR may be the same when a population is constrained and can be fully tested, such as on a cruise ship. Otherwise the IFR calculation will always be lower than the CFR.

Research published last Monday in The Lancet Infectious Diseases estimates that about 0.66 percent of COVID-19 patients who become infected will die. But when undetected cases are not counted, they estimate the coronavirus death rate was 1.38 percent. However, different populations vary in average age, and COVID-19 results in a much higher fatality rate for elderly compared to youth.


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