A woman wearing a protective face mask at an amusement park in Seoul, South Korea April 30, 2020 (Kim Hong-Ji/Reuters)

South Korea has kept its coronavirus outbreak under control with an aggressive regime of testing for infections, tracing the contacts of the infected, and isolating the affected individuals. The nation of 51 million people has had fewer than 11,000 known COVID cases and fewer than 250 known COVID deaths.

Still, that translates to a crude case-fatality rate of over two percent.

It is well known that the true “infection-fatality rate” in every country is significantly lower than the case-fatality rate due to the fact that there are many more undetected cases than undetected deaths.

As Bloomberg Opinion columnist Justin Fox recently reported, the initial round of serological studies that test for coronavirus antibodies — which are being conducted to get a better sense of how many cases are going undetected — point to an infection-fatality rate anywhere between 0.12 percent and 1.08 percent:

So the range of IFRs derived from these surveys so far is 0.12% to 1.08%, and the latter result should probably be given much more credence than the former both because of the false-positives issue described above and the seeming flaws in the calculations used to arrive at 0.12%. The most exhaustive and up-to-date pre-serology-survey estimate of Covid-19’s IFR that I’m aware of, from a peer-reviewed article in Lancet Infectious Diseases by a group of researchers at Imperial College London, is 0.66%. If the IFR of the seasonal flu is 0.04%, these blood surveys show Covid-19 to be anywhere from three times deadlier to 27 times deadlier — and given the incompleteness of current death counts, the true range seems likely to be higher than that.

South Korea seems to be another sign that it is unlikely that the lowest estimate of the infection-fatality rate is correct.

For the rosiest estimate of a 0.1 percent infection-fatality rate to be correct, then there must be 230,000 infections in South Korea that have gone undetected. That seems unlikely in a country where the outbreak is under control, unless asymptomatic cases almost never infect other people. Still, it’s too early to draw a conclusion, and a serological study testing for coronavirus antibodies in South Korea could help shed more light on the virus’s true fatality rate.

Of course, when thinking about fatality rates, it is important to keep in mind that a virus with a seemingly low fatality rate can kill a lot of people. If 30 percent of Americans are eventually infected with a virus that has a 0.25 percent infection-fatality rate, that would result in a death toll of nearly 250,000 people.