At first it can seem surprising that the same virus – which doesn’t seem to have mutated significantly as it has spread – can lead to such widely differing reported mortality rates. And even within one country, the rate appears to change over time. So what’s going on?
Several main factors account for much of the difference we’re seeing – and perhaps the most important come down to simply how we’re counting, as well as testing, cases.
Differing death rates
First, there is confusion about what people mean by “death rate”. This confusion can make countries’ numbers look vastly different, even if their populations are dying at the same rate.
There are, in fact, two kinds of fatality rate. The first is the proportion of people who die who have tested positive for the disease. This is called the “case fatality rate”. The second kind is the proportion of people who die after having the infection overall; as many of these will never be picked up, this figure has to be an estimate. This is the “infection fatality rate”.
In other words, the case fatality rate describes how many people doctors can be sure are killed by the infection, versus how many people the virus kills overall, says Carl Heneghan, an epidemiologist and director of the Centre for Evidence-Based Medicine at the University of Oxford; he is also a GP in recovery from a suspected Covid-19 infection.To see what a difference this makes, consider 100 people who have been infected with Covid-19. Ten of them have it so severely that they go into hospital, where they test positive for Covid-19. The other 90 are not tested at all. One of the hospital patients then dies from the virus. The other 99 people survive.
That would give a case fatality rate of one in 10, or 10%. But the infection fatality rate would be just one in 100, or 1%.