South Korea has reached two seemingly contradictory pandemic milestones. It recorded more than 600,000 new Covid-19 infections on Thursday, the most of anywhere in the world. At the same time, the country has one of the lowest virus death rates globally.

While anywhere else an infection surge such as this would signal an out-of-control outbreak soon to be followed by a spike in fatalities, in South Korea the picture is more complex. 

The sky-high caseload reflects the nation’s consistent deployment of mass testing, largely abandoned by many places as Covid-19 becomes endemic, but a key reason behind the nation’s sliding death rate, according to its virus fighters...