Early reports from January painted a grim picture about just how deadly the coronavirus was. Initially, the World Health Organization estimated that the percentage of infected individuals who die from COVID-19 was 3.4%. That statistic is called the infection fatality rate (IFR) – or colloquially, the death rate – and means that for every hundred people infected with COVID-19, on average, between three and four would die.
As time has gone on, it has become clear that the true risk of death is much lower. The Centers for Disease Control and Prevention currently has a best guess of 0.26% 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.
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.