SEOUL, South Korea — Shin Dong-yun, ​a scientist from the North Korean Institute of Virology, ​rushed to the northwestern border with China in early February. There, he conducted 300 tests, skipping meals to assess ​a stream of people ​so that “the country is protected from the invasion of the novel coronavirus.”

Stories like this, carried in the state-run newspaper Rodong Sinmun, focus attention on one of the stranger oddities surrounding the Covid-19​ pandemic​: How could North Korea claim to not have a single coronavirus case while countries ​around the world stagger under the exploding epidemic?

​North Korea has taken some of the most drastic actions against the virus and did so sooner than most other nations. It sealed its borders in late January, shutting off business with neighboring China, which accounts for nine-tenths of its external trade. It clamped down on the smugglers who keep its thriving unofficial markets functioning. It quarantined all diplomats in Pyongyang for a month. ​The totalitarian state’s singular ability to control the movement of people​ also bolsters its disease-control efforts.

But decades of isolation and international sanctions have ravaged​ North Korea’s public health system, raising concerns that ​it lacks the medical supplies to fight an outbreak, which many fear has already occurred.