The difficulty of calculating the virus’s fatality rate.

With the numbers of coronavirus cases soaring and experts putting out varying estimates of death rates, it can be hard to keep track of what all the numbers mean.

To help, an editor on our science team, James Gorman, tracked down a mathematician who uses math to understand outbreaks of diseases like Ebola, SARS, influenza and now the coronavirus, or Covid-19: Dr. Adam Kucharski at the London School of Hygiene & Tropical Medicine. Here are selections from their edited, condensed exchanges by phone and email.

Image Dr. Adam Kucharski. Credit... Tom Jamieson for The New York Times

To understand the case fatality rate, can we just look at the total number of deaths and the current number of cases?

The problem with just dividing the total number of deaths by the total number of cases is that it doesn’t account for unreported cases or the delay from illness to death. The delay is crucial: If 100 people arrive at hospital with Covid-19 on a given day, and all are currently still alive, it obviously doesn’t mean that the fatality rate is 0 percent. We need to wait until we know what happens to them eventually.

Any deaths will be people who got sick two to three weeks ago, so it’s not simply deaths at the moment divided by cases at the moment. Plus some cases might be missed.

Does the latest estimate of a 3.4 percent fatality rate globally make sense?

Early on, people looked at total current cases and deaths, which, as I said, is a flawed calculation, and concluded that the case fatality rate must be 2 percent based on China data. If you run the same calculation on yesterday’s totals for China, you get an apparent CFR (case fatality rate) of near 4 percent.