TOKYO — With the emergence of the new Omicron variant of the coronavirus late last week, countries across the globe rushed to close their borders to travelers from southern Africa, even in the absence of scientific information about whether such measures were necessary or likely to be effective in stopping the virus’s spread.

Japan has gone further than most other countries so far, announcing on Monday that the world’s third-largest economy would be closed off to travelers from everywhere.

It is a familiar tactic for Japan. The country has barred tourists since early in the pandemic, even as most of the rest of the world started to travel again. And it had only tentatively opened this month to business travelers and students, despite recording the highest vaccination rate among the world’s large wealthy democracies and after seeing its coronavirus caseloads plunge by 99 percent since August.

Now, as the doors slam shut again, Japan provides a sobering case study of the human and economic cost of those closed borders. Over the many months that Japan has been isolated, thousands of life plans have been suspended, leaving couples, students, academic researchers and workers in limbo.

Ayano Hirose has not been able to see her fiancé in person for the past 19 months, since he left Japan for his native Indonesia, just two weeks after her parents blessed their marriage plans.

As Japan has remained closed to most outsiders, Ms. Hirose and her fiancé, Dery Nanda Prayoga, saw no clear path to a reunion. Indonesia had started allowing some visitors, but the logistical challenges were steep. So the couple has made do with multiple daily video calls. When they run out of things to talk about, they play billiards on Facebook Messenger or watch Japanese variety shows together online.

“We don’t want to suffer in pain at the thought of not being able to reunite in the near future,” said Ms. Hirose, 21, who has written letters to the foreign and justice ministries asking for an exemption to allow Mr. Dery to come to Japan. “So we will think positively and continue to hold out hope.”

As the United States, Britain and most of Europe reopened over the summer and autumn to vaccinated travelers, Japan and other countries in the Asia-Pacific region opened their borders only a crack, even after achieving some of the world’s highest vaccination rates. Now, with the emergence of the Omicron variant, Japan, along with Australia, Thailand, Sri Lanka, Singapore, Indonesia and South Korea, are quickly battening down again.

China, which has barred international tourists since the start of the pandemic, is so far still issuing visas for work or diplomatic purposes, although limited flight options and lengthy quarantines have deterred travelers. Taiwan has prohibited nearly all nonresidents from entering since early in the pandemic. Australia, which only recently started allowing citizens and visa holders to travel abroad, said on Monday that it would delay a relaxation of its border restrictions. Sri Lanka, Singapore, South Korea, Indonesia and Thailand have all barred travelers from southern Africa, where the variant was first reported.

Although the true threat of the new variant is not yet clear, Prime Minister Fumio Kishida of Japan told reporters on Monday that he had decided to revoke the relaxations for business travelers and international students in order to “avoid the worst-case scenario.”

The government’s decision to close again reflects its desire to preserve its successes battling the virus and to prevent the kind of strain on the health care system that it experienced over the summer during an outbreak of the Delta variant.

Japan is recording only about 150 coronavirus cases a day, and before the emergence of the Omicron variant, business leaders had been calling for a more aggressive reopening.

“At the beginning of the pandemic, Japan did what most countries around the world did — we thought we needed proper border controls,” Yoshihisa Masaki, director of communications at Keidanren, Japan’s largest business lobbying group, said in an interview earlier this month.

But as cases diminished, he said, the continuation of firm border restrictions threatened to stymie economic progress. “It will be like Japan being left behind in the Edo Period,” Mr. Masaki said, referring to Japan’s isolationist era between the 17th and mid-19th centuries.

Japan had already lagged countries in Southeast Asia, where the economies are dependent on tourism revenues and governments tiptoed out in front in the push to reopen. Thailand had recently reopened to tourists from 63 countries, and Cambodia had just started to welcome vaccinated visitors with minimal restrictions. Other countries, like Malaysia, Vietnam and Indonesia, were allowing tourists from certain countries to arrive in restricted areas.

Wealthier Asian countries like Japan resisted the pressure to reopen. With the exception of its decision to hold the Summer Olympics, Japan has been cautious throughout the pandemic. It was early to shut its borders and close schools. It rolled out its vaccination campaign only after conducting its own clinical trials. And dining and drinking hours remained restricted in many prefectures until September.

Foreign companies could not bring in executives or other employees to replace those who were moving back home or to another international posting, said Michael Mroczek, a lawyer in Tokyo who is president of the European Business Council.

In a statement on Monday, the council said business travelers or new employees should be allowed to enter provided they follow strict testing and quarantine measures.

“Trust should be put in Japan’s success on the vaccination front,” the council said. “And Japan and its people are now firmly in a position to reap the economic rewards.”

Business leaders said they wanted science to guide future decisions. “Those of us who live and work in Japan appreciate that the government’s policies so far have substantially limited the impact of the pandemic here,” said Christopher LaFleur, former American ambassador to Malaysia and special adviser to the American Chamber of Commerce in Japan.

But, he said, “I think we really need to look to the science over the coming days” to see whether a complete border shutdown is justified.

Students, too, have been thrown into uncertainty. An estimated 140,000 or more have been accepted to universities or language schools in Japan and have been waiting months to enter the country to begin their courses of study.

Carla Dittmer, 19, had hoped to move from Hanstedt, a town south of Hamburg, Germany, to Japan over the summer to study Japanese. Instead, she has been waking up every morning at 1 to join an online language class in Tokyo.

“I do feel anxious and, frankly speaking, desperate sometimes, because I have no idea when I would be able to enter Japan and if I will be able to keep up with my studies,” Ms. Dittmer said. “I can understand the need of caution, but I hope that Japan will solve that matter with immigration precautions such as tests and quarantine rather than its walls-up policy.”

The border closures have economically flattened many regions and industries that rely on foreign tourism.

When Japan announced its reopening to business travelers and international students earlier this month, Tatsumasa Sakai, 70, the fifth-generation owner of a shop that sells ukiyo-e, or woodblock prints, in Asakusa, a popular tourist destination in Tokyo, hoped that the move was a first step toward further reopening.

“Since the case numbers were going down, I thought that we could have more tourists and Asakusa could inch toward coming back to life again,” he said. “I guess this time, the government is just taking precautionary measures, but it is still very disappointing.”

Mr. Dery and Ms. Hirose also face a long wait. Mr. Dery, who met Ms. Hirose when they were both working at an automotive parts maker, returned to Indonesia in April 2020 after his Japanese work visa expired. Three months before he departed, he proposed to Ms. Hirose during an outing to the DisneySea amusement park near Tokyo.

Ms. Hirose had booked a flight to Jakarta for that May so that the couple could marry, but by then, the borders were closed in Indonesia.

“Our marriage plan fell apart,” Mr. Dery, 26, said by telephone from Jakarta. “There’s no clarity on how long the pandemic would last.”

Just last week, Mr. Dery secured a passport and was hoping to fly to Japan in February or March.

Upon hearing of Japan’s renewed border closures, he said he was not surprised. “I was hopeful,” he said. “But suddenly the border is about to close again.”

“I don’t know what else to do,” he added. “This pandemic seems endless.”

Reporting was contributed by Hisako Ueno and Makiko Inoue in Tokyo; Dera Menra Sijabat in Jakarta, Indonesia; Richard C. Paddock in Bangkok; John Yoon in Seoul; Raymond Zhong in Taipei, Taiwan; and Yan Zhuang in Sydney, Australia.