While the virus is flaring elsewhere in China, including in its northern regions, officials say they’re unable to establish a chain of transmission. Their suspicions have instead settled on their neighbour, with authorities urging residents living by the Yalu River that runs between the two countries to close their windows on days with southerly winds, according to a government notice. They’re also being asked to go for more frequent testing, said a Dandong resident, who asked not to be named for fear of reprisal.

Residents suspect authorities are considering the possibility that the virus is being carried through the air from North Korea, the person said. The isolated nation is experiencing a full-blown crisis with suspected cases topping 4 million since late April, according to the state-run Korean Central News Agency.

Representatives from the Dandong and Liaoning CDCs said they had no details about the virus spreading through the air when reached by phone.

Kim orders lockdown after North Korea reports first Covid case

Elsewhere, Beijing reported eight infections for Tuesday. The capital started to roll back more virus restrictions on Monday, including resuming public transport and allowing people to move around more freely. Shanghai recorded 15 infections on Tuesday. Relief over last week’s lifting of a gruelling lockdown has been curtailed somewhat by an uptick in cases outside government-mandated quarantine, with four cases found in the community yesterday.

Beijing sees no new Covid cases outside quarantine in four days

Not everyone is convinced about the risks from North Korea. Many Chinese social media users said that the suggestion that the coronavirus can travel in the air for hundreds of metres is unscientific. Research shows that infections through airborne transmission is unlikely over long distances, particularly in outdoor settings without repeated exposure.

The mysterious flare-up underscores the challenges China faces in adhering to its Covid Zero goal as the virus becomes ever more transmissible and contact tracing breaks down. China’s border towns have been subject to some of the harshest containment measures of the pandemic as the virus persistently seeps in. Ruili, a city on China’s border with Myanmar, has been in lockdown intermittently for more than 160 days, according to local media reports.

North Korea and China share a porous 1,300 kilometre border, in some areas separated by the Yalu River that is less than one kilometre wide in parts of Dandong.

The city is a key trading hub for the two countries. Around 70% of North Korea’s foreign trade went through Dandong prior to the pandemic. Rail freight between Dandong and neighboring Sinuiju city in North Korea has been suspended for much of the pandemic. BM