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– Chinese authorities plan to close off Wuhan — a city of more than 11 million people and the epicenter of a mysterious, pneumonialike virus that has already spread halfway around the world — canceling planes and trains leaving the city beginning Thursday, and suspending buses, subways and ferries within it.

The announcement, shared on Chinese state media just hours before it was to take effect, was a significant escalation from just the day before, when authorities had urged people not to travel to or from the central Chinese city but had stopped short of shutting down transportation. The new virus, which first emerged at the end of December, has killed at least 17 people and sickened more than 470.

Authorities said that the measures, which would take effect at 10 a.m., were needed to “effectively cut off the transmission of the virus, resolutely curb the spread of the epidemic, and ensure the safety and health of the people.”

They said they would announce an end date for the restrictions separately.

The transportation shut-off, announced soon after 2 a.m., could upend the travel plans of millions of Chinese citizens, who travel in huge numbers during the Lunar New Year holiday. The government said it would shut down airports and train stations to departures, and urged residents not to leave the city — a major transportation hub — unless they had an urgent reason to do so.

The Lunar New Year in China is the world’s largest annual migration of people, with hundreds of millions of travelers fanning out across the country and the world, and hundreds of billions of dollars spent on hotels, restaurants and shopping.

Now, with the new coronavirus, the mass migration is also an epidemiologist’s nightmare.

Authorities are scrambling to control the disease, which has spread around the region, even reaching North America. The World Health Organization met Wednesday to discuss whether to declare the outbreak an international health emergency, which would escalate the global response.

International officials are stepping up screening at major airports. In the United States, all airline passengers from Wuhan, regardless of destination, would be funneled to one of five screened airports.

Before the virus emerged, the Chinese government had estimated that domestic travelers would make 3 billion trips over the holiday period, also known as the Spring Festival. But on Tuesday, a senior health official delivered a stark warning: The tide of travel during the holiday would make it more difficult to contain the outbreak.

“Even as we work harder, it will increase,” the official, Zeng Guang, said at a news conference, though he said that after the Lunar New Year it would quickly drop off again.

Li Bin, a deputy head of China’s health commission, also said on Wednesday that the virus could mutate and spread more easily.

Many Chinese have already canceled their travel plans, forgoing vacations and what for some is their only chance to return home for family reunions during the year. The Lunar New Year, a weeklong holiday, begins on Friday, when the country says farewell to the Year of the Pig and welcomes the Year of the Rat.

“After we heard how bad the situation was on Monday, we held a family meeting and decided that it just wasn’t worth the risk,” said Yan Chaowei, 32, a housewife in Shanghai who was planning on taking a seven-hour bullet train to her family home in southeastern Jiangxi province.

“It just wouldn’t be a relaxing trip, especially with a small child,” she added. “When we finally made the decision to stay home, we sighed with relief.”

Even some working in the travel industry were nervous. Flight attendants at Cathay Pacific, Hong Kong’s leading airline, publicly lobbied the company to allow them to wear masks during all flights, a request that was granted on Wednesday.

“It will be too late and too painful for all of us and the Company to wait until one of our own is infected,” the airline’s union for flight attendants said in a statement. “The damage caused will be catastrophic.”

In Wuhan, which is popular among tourists for its colonial architecture, spicy noodles and proximity to the Yangtze River, authorities had already issued a ban on large public gatherings and performances at hotels and sightseeing destinations. They had also announced that all locals were required to wear masks in public to help prevent the spread of the virus.

To encourage travelers to stay away from Wuhan, tour companies promised penalty-free refunds for hotel bookings and air and train tickets to and from the city. Travel operators suspended itineraries with stops there, raising concerns of a slump during what is usually one of the most lucrative weeks of the year.

For Chinese companies, the outbreak could deal yet another blow at a time of slowing economic growth.

According to official estimates, Chinese spent $74 billion on travel and $145 billion on shopping and food during the Spring Festival holiday last year.

The holiday is also one of the most profitable periods for the Chinese box office, but there are concerns that potential moviegoers might stay home to avoid sitting in enclosed spaces with strangers.

Many also take advantage of the weeklong holiday to travel abroad, particularly in the region, but may be forced to change their plans this year.

Tsai Ing-wen, Taiwan’s president, said Wednesday that all travel by tour groups between Wuhan and Taiwan would be suspended, a day after her government confirmed the island’s first case of the new coronavirus.

In Hong Kong, one of the biggest travel agencies said it was cutting all tour groups to, or passing through, Wuhan until the end of March, according to news reports.

Airports around the world have stepped up screening measures for travelers arriving from Wuhan.

The health authorities in Hong Kong are also requiring airlines to distribute health declaration forms and to make facemasks and antiseptic wipes available at boarding gates for passengers arriving from Wuhan.

Experts have warned that with the travel rush already underway this week, the virus’ continued spread may be inevitable.

Zhong Nanshan, a prominent scientist who is leading a government-appointed panel of experts working to control the outbreak, singled out the cramped train rides that many Chinese have to endure during the holiday as potential hotbeds of transmission.

Zhu Niancheng, 19, a chemistry major at a university in Wuhan, appeared to be heeding that advice Wednesday as he sat on a suitcase outside a Beijing train station smoking a cigarette.

“Don’t worry, I’m not going to Wuhan,” he said, as a phalanx of People’s Liberation Army soldiers in green uniforms and black facemasks marched behind him.

Asked if he was concerned about his classmates back in Wuhan, the teenager laughed and said, “I’m not really afraid. We just make fun of each other on WeChat, like ‘Yo, you still alive?’ ”