So far, when people have quoted a fatality rate of around 2% for the current coronavirus outbreak, they have done so by making an understandably intuitive calculation: dividing the death toll by the total number of confirmed cases. If we do this for China’s latest (Feb. 6) figures—636 deaths divided by 31,161 confirmed cases—we get what appears to be a 2% fatality rate.

But this could very well give an underestimation of the fatality rate, for a simple reason: time. Since it takes time to die from the coronavirus, and since the number of cases is growing so quickly, the number of confirmed cases on a given day will be much higher than the number of people who have been confirmed infected and progressed to death. In a similar way, any delays between someone dying and the death being reported could cause an underestimation of the fatality rate. This happened during the SARS and Ebola outbreaks, according to a paper on potential biases in calculating fatality risks published in the journal PLOS Neglected Tropical Diseases. If there’s a delay in reporting deaths, then dividing total deaths by confirmed cases would mean including people who will die but haven’t yet, or have died but haven’t been reported.

The bottom line is that there are lots of factors that complicate the picture beyond simply dividing the number of deaths by total confirmed cases. What cases are diagnosed? Are mild cases being undercounted, so that there’s a disproportionate number of severe cases in the total count? How much time on average elapses between diagnosis and death? Is there a delay in reporting deaths? Until there is sufficient information to answer these questions, it’s dangerous to conclude the deadliness of China’s coronavirus.


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