In a press briefing on March 3, 2020, the Director-General of the World Health Organization (WHO), Dr. Tedros Adhanom Ghebreyesus, reported a 3.4% mortality rate from COVID-19. But how reliable is this figure? We take a look at what British experts have to say on the matter.
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In the media briefing, Dr. Ghebreyesus reported a total of 90,893 cases of COVID-19 globally, with 3,110 resulting in death.
Dr. Ghebreyesus also made some comparisons with the common flu in his statement. These included the facts that COVID-19 “does not transmit as efficiently” as influenza, there are no vaccines and therapeutics for COVID-19, and containment strategies could work for COVID-19 but would not be possible for the flu.
An additional and important observation that Dr. Ghebreyesus made regards the severity of the two illnesses. Unlike the flu, nobody is immune to COVID-19. As a result, more people are prone to infection, and many are susceptible to “severe disease.”
“Globally, about 3.4% of reported COVID-19 cases have died. By comparison, seasonal flu generally kills far fewer than 1% of those infected.”
– Dr. Tedros Adhanom Ghebreyesus
But how was this mortality rate calculated? And what are the unique challenges of determining the death rate during an epidemic?
Some of the world’s leading health experts have weighed in. Below, we summarize their opinions.
“It is surprisingly difficult to calculate the ‘case fatality ratio,’ or death rate, during an epidemic,” says John Edmunds, a professor in the Centre for the Mathematical Modelling of Infectious Diseases at the London School of Hygiene & Tropical Medicine in the United Kingdom.
This difficulty is due to the long period between the onset of the illness and the fatality, explains Prof. Edmunds.
For COVID-19, this length of time is 2–3 weeks or more, he says. Therefore, to calculate the case fatality rate, we should use the number of confirmed cases from a few weeks ago, rather than at the present time.
Experts define the case fatality rate as “the ratio of deaths occurring from a particular cause to the total number of cases due to the same cause.”
But, continues Prof. Edmunds, in the case of a “rapidly expanding epidemic,” the number of cases from a few weeks ago will always be much smaller than the current one, so “the true case fatality ratio will be higher.”
On the other hand, another bias evens the scale in the opposite direction.
“We do not report all the cases,” says Prof. Edmunds. “In fact, we only usually report a small proportion of them. If there are many more cases in reality, then the case fatality ratio will be lower.”
In conclusion, estimating the true case fatality ratio is “tricky,” says the researcher.
“What you can safely say […] is that if you divide the number of reported deaths by the numbe