How deadly is the coronavirus?

Early reports from January painted a grim picture about just how deadly the coronavirus was. On March 3, the Director-General of the World Health Organization (WHO) said that "Globally, about 3.4% of reported COVID-19 cases have died." At the time, it was unclear whether the number of confirmed cases was very different from the number of total cases, so the WHO used confirmed cases to calculate how deadly the coronavirus was. This caused a lot of confusion among the media and the public.

But as researchers like us have learned more about the spread of the virus, we have discovered that the total number of infected people is far greater than the number of confirmed cases. When deaths from COVID-19 are divided by the total number of cases – not just reported cases – you get a statistic called the infection fatality rate (IFR), or colloquially, the death rate. The Centers for Disease Control and Prevention currently has a best guess of 0.65% for the IFR. But current estimates fall anywhere between 0.2% and 1%, a surprisingly large range when calculating the infection fatality rate should be as simple as dividing the number of deaths by total infections. And these estimates are changing all the time. In fact, in the time it took to write this article, the CDC changed its best estimate of the fatality rate from 0.26% to 0.65%.

We are two researchers who take a mathematical approach to solving epidemiological and biomedical problems. Back in early March, we published a paper showing that millions more people had been infected with COVID-19 than official case counts reflected. But when we tried to use our results to calculate IFR in the U.S., we encountered firsthand just how difficult it is to do.

To calculate the infection fatality rate, researchers need to know three things: the number of infections, the number of deaths from infections and which deaths go with which infections. But finding these numbers is far harder than it might seem and these difficulties explain why there has been, and continues to be, so much uncertainty regarding this important number.

How many infections?

Knowing how many people have been infected with the coronavirus is the first step to estimating the fatality rate.

The number of officially reported cases reflects only the number of diagnosed cases which is far less than the real number of people who have been infected.

Since health officials can't test everyone, one way to estimate the rate of infection in a population is to test a smaller group of people for signs of previous infection, regardless of whether they have had symptoms. If the smaller group is chosen in a way that makes it demographically representative of the larger population, then researchers can assume the infection rates they find in their test groups are close to the actual population-wide numbers.

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(This article is republished from The Conversation under a Creative Commons license. The Conversation is an independent and nonprofit source of news, analysis and commentary from academic experts.)

Justin Silverman

Pennsylvania State University

and Alex Washburne

Montana State University

(THE CONVERSATION)