COVID-19: the Case Fatality Rate (CFR) and Infection Fatality Rate (IFR)
Reader Staale A writes with a link to OurWorldInData.org, which among other things does a great job of explaining the CFR (Case Fatality Rate) and IFR (Infection Fatality Rate), the latter being a total unknown. It is by far the best discussion I have seen of COVID-19 infection and mortality—excellent analysis and logic.
That page should be required reading for every member of the media, which is wrongly characterizing so many things, including COVID-19. And for those so inclined to read and understand the article, you will acquire a critical skill in understanding just how bad the media is at explaining what is going on.
OurWorldInData.org: What do we know about the risk of dying from COVID-19?
Case Fatality Rate (CFR) = (Number of deaths from disease) / (Number of diagnosed cases of disease)
Crude Mortality Rate (CMR) = (Number of deaths from disease) / (total population)
Infection Fatality Rate (IFR) = (Number of deaths from disease) / (number infected)
The various graphs are the best I’ve seen in coverage:
OurWorldInData.org graphs by age, country, etc
For those who are still saying “it’s just the flu”: infection rates are escalating (this thing is just getting started) and the Case Fatality Rate (CFR) is so far 12 times higher for all ages. The graph below should make anyone in a higher-risk group concerned enough to take every possible precaution.
The key point is that the “case fatality rate”, the most commonly discussed measure of the risk of dying, is not the answer to the question, for two reasons. One, it relies on the number of confirmed cases, and many cases are not confirmed; and two, it relies on the total number of deaths, and with COVID-19, some people who are sick and will die soon have not yet died. These two facts mean that it is extremely difficult to make accurate estimates of the true risk of death.
What we want to know isn’t the case fatality rate: it’s the infection fatality rate
Remember the question we asked at the beginning: if someone is infected with COVID-19, how likely is it that they will die? The answer to that question is captured by the infection fatality rate, or IFR.
However, the total number of cases of COVID-19 is not known. That’s partly because not everyone with COVID-19 is tested..... and despite what some media reports imply, the CFR is not the same as – or, probably, even similar to – the IFR. Next, we’ll discuss why.
The case fatality rate isn’t constant: it changes with the context
Sometimes journalists talk about the CFR as if it’s a single, steady number, an unchanging fact about the disease. This is a particularly bad example from the New York Times in the early days of the COVID-19 outbreak. But it’s not a biological constant; instead, it reflects the severity of the disease in a particular context, at a particular time, in a particular population...
Importantly, this means that the number of tests carried out affects the CFR – you can only confirm a case by testing a patient. So when we compare the CFR between different countries, the differences do not only reflect rates of mortality, but also differences in the scale of testing efforts.
What we do know if that the mortality risk is higher for older populations and those with underlying health conditions such as cardiovascular disease, diabetes and respiratory disease – we look at some preliminary evidence for this in our full coverage of the COVID-19 pandemic here.