In response to a single case of the coronavirus, Chinese authorities locked down a southern border city of more than 200,000 people this week, barring the entry of all goods and people. After a cluster of new cases in northwestern China, officials also sealed off a city of 13 million, ordering all residents to stay inside.

The extreme response underlines China’s hypervigilance as Beijing prepares to host the Winter Olympics in February amid new local cases of the omicron variant.

The city of Dongxing, which borders Vietnam in China’s southern Guangxi province, on Wednesday ordered all households to quarantine at home until further notice after a resident tested positive during a routine screening, according to state broadcaster CCTV. Schools, public transportation and most businesses, except for supermarkets and pharmacies, were temporarily shuttered as authorities launched a campaign to test everyone in the city.

Customs processing in the city, the entry point for a million tons of goods annually from Vietnam, was also halted while the Chinese Embassy in Hanoi asked Chinese citizens in Vietnam not to return by land.

Authorities in Xi’an in Shaanxi province locked down the city of 13 million, allowing only one person per household to go outside every other day to buy supplies — measures reminiscent of the unprecedented lockdown of the city of Wuhan in early 2020.

Officials in Xi’an say they are facing the “double threat” of new coronavirus cases as well as an outbreak of hemorrhagic fever, a seasonal disease caused by the hantavirus. “Currently, the situation of pandemic prevention and control remains complicated and grim,” said Zhang Fenghu, deputy secretary general of the city government, announcing the lockdown after authorities reported 52 new coronavirus cases Wednesday. The city has reported a total of 143 cases since Dec 9.

China’s pursuit of a stringent “zero covid” policy has resulted in increasingly strict border controls and quarantines and frequent lockdowns across the country. According to Vietnam’s state-controlled Hanoi Times, more than 6,000 trucks carrying fruit have been stranded along the Chinese-Vietnamese border for several weeks.

Seven omicron cases have been detected in China over the past two weeks, just before Lunar New Year when millions of residents would normally travel home to spend the holiday with family. Officials — wary of forcing pandemic-weary residents to miss a third Lunar New Year in a row — are strongly urging citizens to “celebrate on the spot” and avoid going home. On Monday, Lei Zhenglong, deputy director of China’s National Health Commission, ordered officials not to let the country’s defenses against omicron relax “in the slightest.”

In Xi’an, authorities were struggling to manage angry and frustrated residents as citywide testing got underway. On Tuesday, the city’s “health code” system, a QR-based tracking app that residents must show before entering public spaces or traveling, had collapsed after being overloaded with traffic.

On social media, residents complained that they were not able to ride public transportation or get into their apartment compounds because of the health code system’s failure. In lieu of the app, which includes residents’ vaccine status and recent coronavirus test results, some companies resorted to having employees sign written statements declaring they are not infected.

Authorities have required all residents leaving the city to present an official letter from local authorities allowing them to board departing trains. One resident stranded in the Xi’an North Railway Station described the confusing and contradicting array of rules on the social network Weibo, in a post that was later erased.

“Waited an hour for a nucleic acid test. After buying my ticket, I was told I would need a special certificate from my local neighborhood office,” the post began. “Took an hour bus to that office where someone told me no certificate was needed. Took another hour bus back to the train station where I was told to go back to my neighborhood office. Went back. By then, my nucleic acid test was no longer valid. Waited in line again for another test. Took the result to the neighborhood office who told me to go to the subdistrict office for a letter. Staff there said the leaders were discussing. I waited and waited and waited. Then my coronavirus test expired again.”

Another Weibo user in Xi’an expressed similar sentiments. “These days I feel distress, anger and helplessness,” the person wrote.