An extensive buildup of barriers along China’s 3,000-mile southern border is under way, according to public documents, official statements and interviews with residents, ostensibly to battle Covid-19 but with likely long-lasting ramifications on trade and travel.

The small Chinese city of Ruili, in the far south next to Myanmar, has seen a major construction project in the past two years. It is a border fence equipped with barbed wire, surveillance cameras and sensors.

Farther...

An extensive buildup of barriers along China’s 3,000-mile southern border is under way, according to public documents, official statements and interviews with residents, ostensibly to battle Covid-19 but with likely long-lasting ramifications on trade and travel.

The small Chinese city of Ruili, in the far south next to Myanmar, has seen a major construction project in the past two years. It is a border fence equipped with barbed wire, surveillance cameras and sensors.

Farther east, along China’s border with Vietnam, a 12-foot-high fence went up abruptly last year. It stops Vietnamese locals from heading to Chinese villages to harvest corn or sell medicinal herbs, and it looks like a prison, said Sung A Ho, a hotelier in Vietnam’s mountainous Lao Cai province.

The avowed purpose is to fight the spread of Covid-19 by limiting the entry of traders, workers and smugglers. The Southern Great Wall, people on social media are calling it. State media outlets have dubbed it the Anti-Covid Great Wall.

While some other countries try to transition toward living with Covid-19, China determinedly maintains a zero-Covid strategy, especially with the Beijing Winter Olympics starting this week. It does so not only through lockdowns and mass testing but also, increasingly, by walling off its neighbors.

The strategy means that along China’s long southern border, life is changing in ways likely to last long beyond the pandemic, with trade getting cumbersome and with control of people’s movements tightened.

China’s anti-Covid efforts put an end to formerly easy crossings between the Chinese city of Ruili and the town of Muse in Myanmar.

Photo: TPG/Getty Images

The efforts are part of a wider drive by China to secure its borders, facilitate infrastructure projects and prevent refugees from crossing into China, said Karin Dean of Tallinn University in Estonia, who studies the border dynamics between China and Myanmar.

China’s calls to guard against a Covid-19 spread via the border are emphatic. In an August letter to villagers in border areas of Yunnan province, Chinese leader Xi Jinping urged locals to “safeguard the sacred land.” He told officials and civilians to unite to build an unbreachable barrier.

In Guangxi, a region bordering Vietnam, party officials urged cadres to “race against time, go all out, resolutely win the battle against the pandemic and defend the ‘south gate’ ” of China. The Communist Party chief of Yunnan province called for a mind-set of “facing death unflinchingly” to reassure the party leadership and Mr. Xi.

Yunnan, which borders Myanmar, Vietnam and Laos, earmarked a half-billion-dollar fund last year to fortify security barriers at the border. The province’s governor said in January that 100,000 officials, police officers, soldiers and civilians have been patrolling the border.

A Journal review of public records shows that in the past two years, China has built or strengthened at least 285 miles of fencing along its borders, most of it in the south. The actual figures are likely higher because not all local governments disclose this type of spending. At a wire-making center in Hebei province, some manufacturers bill their products as “border Covid-prevention wire.”

A high fence along China’s border snakes through the mountains near Vietnam’s Binh Lieu district.

Photo: Lam Le

In the Vietnamese coastal province of Quang Ninh, home to small farming communities belonging to the Dao ethnic group, a sturdy fence with pillars, sharp-edged metal grills and coiled barbed wire runs across hilltops along the Chinese border as far as the eye can see. Cameras and lights appear at intervals atop the recently built blue-painted structure, with a narrow paved path on the Chinese side.

The lights come on at night, said 31-year-old Duong On, a Vietnamese farmer who lives in a settlement at the base of the hill. People from the Vietnam side can no longer venture across to cut trees, Mr. Duong said, and Chinese farmers are no longer seen grazing their water buffaloes on the Vietnam side.

A border fence means Phun Thi Ha can no longer walk from Vietnam into China to cut grass for use in making brooms.

Photo: Lam Le

On a visit to villages along that stretch, long fences were visible at different points. Some appeared rudimentary—a few layers of stacked barbed wire—while others were more elaborate, like the one near Mr. Duong’s home. Residents said Chinese authorities are continuing to build fences every day.

Phun Thi Ha, 65, used to cross into China to source star anise spices from the fruit of evergreen trees and cut grass for making brooms. A fence now blocks her path. A few miles from a trail, where locals say cross-border travel was once common for work or to smuggle chicken feet, pork intestines and other frozen food, a fence now snakes along the border.

“The fence came up as a Covid-19 barrier and also so that goods can’t cross,” said Doong A Tai, a resident.

China put barriers on some parts of its borders long before Covid-19, not only near North Korea in the northeast and Xinjiang in the far west, but also in the south, where smuggling is a headache. But the extent to which the country has expanded fortifications all along the southern border during the pandemic has gone largely unnoticed outside the region.

In some spots, it includes the kind of surveillance common in China’s big cities. Xiaoguangnong, a tiny village of only 260 people close to Myanmar, has a facial-recognition system to distinguish locals from outsiders, according to state media.

Besides the southern border, several northern Chinese regions adjoining Mongolia and Russia have been fortifying border fences over the past two years, public records show. The efforts there often focus on strengthening existing fences rather than building new ones.

China’s Ministry of Foreign Affairs said in response to questions that fortifying borders is a widely accepted international practice, and that the fencing is helping to prevent cross-border transmission of Covid-19.

Vietnam’s government didn’t respond to requests for comment, nor did that of Myanmar.

The southern Chinese city of Ruili, a jewelry-trading hub, has taken a heavy hit in China’s fight against Covid-19. The local party secretary was stripped of his duties last April for failure to keep the disease suppressed.

Ruili officials said in October that just since July, 716 people who entered from neighboring Myanmar, both Chinese and non-Chinese, had tested positive for Covid-19. The city has imposed lockdown after lockdown, lifting the most recent one late last year. In the first nine months of 2021, Ruili’s economy contracted 8.4% from a year earlier. Many residents have left.

Ruilli has spent much of the past two years building a strong border fence and a buffer zone. An elaborate system now allows cross-border trade to take place with little or no human contact, according to local people and to state media reports.

Trucks from Myanmar must stop at the Chinese border. Their cargoes have to be sanitized and remain on the Myanmar side for 48 hours. Then, robots and cranes move the cargo onto Chinese trucks. Chinese drivers take the goods across the border, where they are sanitized again and held for 24 more hours before they can be cleared for distribution inside China.

Vietnamese workers in protective suits disinfect a container truck at Huu Nghi border gate connecting with China.

Photo: kham/Reuters

“At least now we can restart our business,” said Chen Yunzhong, a 56-year-old businessman who imports fruits to China from Myanmar. “There was a period of time that I thought I had to do something else to make a living.”

Ruili’s government has put thousands of police and ordinary citizens on duty to watch the border 24 hours a day, according to local official media.

A farmer in his 30s said he was among those placed on night duty to watch for illegal crossings—work that isn’t paid. He said he hasn’t been able to sell his garden trees and other products because so many people have left the city. The farmer said he received Covid-19 subsidies totaling $470 last year, but without a steady income, he has taken to digging up taro root and boiling it with vegetables in order to eat.

For China, keeping trade flowing at border points is crucial, both to ensure livelihoods and to bring in needed materials. A railway linking Kunming in Yunnan province to Vientiane, Laos—a project of China’s Belt and Road trade initiative—carries cargo but won’t take passengers across the border until after the pandemic, according to state media.

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The new barriers and other controls appear likely to forever change the relationships among many communities along the border. Ruili’s restrictions cut off formerly easy crossings from the Myanmar city of Muse, said Sai Khin Maung, an executive of a trade-focused chamber of commerce in Muse.

Because all trade in watermelons, mangoes, corn and other produce must now flow through official border gates, taking much longer, the produce often rots, Mr. Maung said. “We cannot express the impact on us,” he said.

China has been trying to control its border with Myanmar for years to curb activities like smuggling and drug trafficking, said David Brenner, a University of Sussex lecturer and author of a book on Myanmar’s borderlands.

“Covid-19 might be the official justification that China is giving for building the buffer zone now,” he said. “But that intention started long before and will govern things long after the pandemic ends.”

—Natasha Khan contributed to this article.

Write to Liyan Qi at liyan.qi@wsj.com and Keith Zhai at keith.zhai@wsj.com