When there are people who have the disease but are not diagnosed, the CFR will overestimate the true risk of death. One of them would tend to make the CFR an overestimate – the other would tend to make it an underestimate. There are two reasons why we would expect the CFR not to represent the real risk. 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? When we talk about the CFR of a disease, we need to talk about it in a specific time and place – the CFR in Wuhan on 23rd February, or in Italy on 4th March – rather than as a single unchanging value. This shows that what we said about the CFR generally – that it changes from time to time and place to place – is true for the CFR of COVID-19 specifically. By 1st February, the CFR in Wuhan was still 5.8% while it was 0.7% across the rest of China. You can also see that the CFR was different in different places. The WHO reported that that was because “the standard of care has evolved over the course of the outbreak”. You can see that in the earliest stages of the outbreak the CFR was much higher: 17.3% across China as a whole (in yellow) and greater than 20% in the center of the outbreak, in Wuhan (in blue).īut in the weeks that followed, the CFR declined, reaching as low as 0.7% for patients who first showed symptoms after February 1st. It shows the CFR for COVID-19 in several locations in China during the early stages of the outbreak, from the beginning of January to 20th February 2020. You can see that in the chart below, first published in the Report of the WHO-China Joint Mission on Coronavirus Disease 2019 (COVID-19), in February 2020. This was clear right from the start of the pandemic. The case fatality rate of COVID-19 is not constant. If the crude mortality rate really was 2.7%, then the case fatality rate was much higher – it would be the percentage of people who died after being diagnosed with the disease. This means the crude mortality rate was 2.7%.īut 2.7% is often misreported as the case fatality rate – which is wrong, because not everyone in the world was infected with the virus that caused the Spanish flu. 2 That would have been 2.7% of the world population at the time. One estimate for the death toll of the Spanish flu, by Johnson and Mueller (2002), is that the pandemic killed 50 million people. A common example is the Spanish flu pandemic in 1918. Unfortunately, writers sometimes confuse case fatality rates and crude death rates. For instance, if there were 10 deaths in a population of 1,000, the crude mortality rate would be, or 1%. It’s calculated by dividing the number of deaths from the disease by the total population. The crude mortality rate – sometimes also called the crude death rate – measures the share among the entire population that have died from a particular disease. The “crude mortality rate” is another very simple measure which, like the CFR, gives something that might sound like the answer to the question “if someone is infected, how likely are they to die?”.īut, just as with CFR, it is actually very different. That means that it is not the same as – and, in fast-moving situations like COVID-19, probably not even very close to – the true risk for an infected person.Īnother important metric, which should not be confused with the CFR, is the crude mortality rate. So if 10 people have died, and 100 people have been diagnosed with the disease, the CFR is, or 10%.īut it’s important to note that it is the ratio between the number of confirmed deaths from the disease and the number of confirmed cases, not total cases. You take the number of people who have died from the disease, and you divide it by the total number of people diagnosed with the disease. It is relevant and important, but far from the whole story. The CFR is not the same as the risk of death for an infected person – even though, unfortunately, journalists sometimes suggest that it is. This measure is sometimes also called case fatality risk or case fatality ratio. In the media, it is often the “case fatality rate” that is talked about when the risk of death from COVID-19 is discussed.
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