If the case fatality rate does not tell us the risk of death for someone infected with the disease, what does it tell us? And how does the CFR compare with the actual mortality risk?

There are two reasons why we would expect the CFR not to represent the real risk. One of them would tend to make the CFR an overestimate – the other would tend to make it an underestimate.

When there are people who have the disease but are not diagnosed, the CFR will overestimate the true risk of death. With COVID-19, we think there are many undiagnosed people.

As we saw above, in our discussion on the difference between total and confirmed cases (here), we do not know the number of total cases. Not everyone is tested for COVID-19, so the total number of cases is higher than the number of confirmed cases.

Whenever there are cases of the disease that are not counted, the probability of dying from the disease is lower than the reported case fatality rate. Remember our imaginary scenario with 10 deaths and 100 cases. The CFR in that example is 10% – but if there actually 500 cases (and we’ve simply missed 400 of them due to lack of testing), then the real risk (the IFR) is just 2%.

Or to summarize in one sentence. If the number of total cases is higher than the number of confirmed cases, then the ratio between deaths and total cases is smaller than the ratio between deaths and confirmed cases.12

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.

A second consideration is especially important in the early stages of an outbreak: When some people are currently sick and will die of the disease, but have not died yet, the CFR will underestimate the true risk of death.

In ongoing outbreaks, people who are currently sick will eventually die from the disease. This means that they are currently counted as a case, but will only eventually be counted as a death too. With the COVID-19 outbreak, it can take several weeks for people to go from first symptoms to death.

This means that the CFR in the early stages is an underestimate of what it will be when the outbreak has run its course.