Moving to counter the spreading new coronavirus outbreak, the Trump administration said Friday it would bar entry by most foreign nationals who had recently visited China and put some U.S. travelers under a quarantine as it declared a rare public-health emergency.

The temporary restrictions followed announcements by American Airlines, Delta Air Lines and United Airlines that they would suspend air service between the United States and China for several months.

On Sunday, the U.S. will begin funneling all flights to the U.S. from China to seven major airports, including Seattle-Tacoma International, where passengers can be screened for illness.

The administration’s action will restrict all foreign nationals who have been to China in the past 14 days from entering the U.S. The restriction does not include immediate family members of U.S. citizens and permanent residents. Nearly 3 million Chinese residents traveled to the U.S. in 2018, according to federal data based on travel records.

Chinese officials said Saturday there had been an additional 46 deaths in the country, the most so far in a 24-hour period, raising the death toll to 259. It said confirmed infections had grown to nearly 12,000, from 1,300 a week earlier.

About 100 cases have been confirmed in 21 other countries, including seven reported cases in the U.S. Russia, Italy and Britain each reported their first infections Friday, two from each country.

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The travel restrictions and the airline’s announcements showed how rapidly concerns about the virus have escalated into a grave test of the global economy, for which there is no recent precedent. Three weeks after the first virus-related death was reported, China has found itself increasingly cut off from its biggest trading partner, the United States, and many other nations.

The travel disruption sent shocks through the stock market and rattled industries that depend on the flow of goods and people between the world’s two largest economies. Planning was upended for companies across a vast global supply chain, from Apple to John Deere, the tractor company.

The S&P 500 suffered its worst loss since October, falling 1.8 percent, as the spread of the virus — and the increasingly urgent efforts by companies and governments to contain it — fanned fears of an economic slowdown.

Dr. Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, said at a Washington briefing that the actions were being taken because there were “a lot of unknowns” surrounding the novel coronavirus and its transmission path. Unlike influenza, which is fairly predictable in terms of infection and mortality, Fauci said there was not the same certainty about the rate and path of the transmission. The number of cases has “steeply inclined with every day,” Fauci said.

There are seven cases of the new coronavirus in the U.S. and all were travelers except for a Chicago man who caught it from his wife, who had been in China. The first confirmed case in the U.S. is a Snohomish County man who remains hospitalized in Everett.

In addition to Sea-Tac, the other airports that travelers from China will be sent to are John F. Kennedy International in New York, Los Angeles International and San Francisco International in California, Chicago O’Hare International, Hartsfield-Jackson Atlanta International in Georgia, and Daniel K. Inouye International in Hawaii.

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The CDC is already conducting health screenings for the new coronavirus at Sea-Tac. The airport is one of 20 nationwide where the federal health agency has what is known as a “quarantine station,” according to Perry Cooper, a spokesman for the Port of Seattle. Despite the name, people are not quarantined in the station, inside the Customs area, but are given screenings by CDC staff who work there all the time as various illnesses break out around the world. The quarantine period is 14 days.

Cooper said he did not know where people judged to be at risk of coronavirus would be quarantined. Federal officials will determine that.

Nearly 200 people evacuated this week from Wuhan were taken to a military base in Southern California. A Joint Base Lewis-McChord spokeswoman said the base and the Yakima Training Center “are no longer being considered.”

Tara Lee, a spokeswoman for Gov. Jay Inslee, said his office was working with the federal government to help find appropriate locations.

To address the outbreak, China has extended the Lunar New Year holiday, which was to have ended Thursday, into next week. In cities across the country, including those far from the center of the outbreak, there were eerie scenes Friday of all-but-empty streets and highways, closed shops, trains without passengers and nearly deserted public spaces that are normally packed.

The slowdown in activity has raised fears that essential supplies, including food, will run short, which the government insists it will not allow to happen.

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And it is unclear when China’s economic engine — a huge producer of both consumer goods and industrial components — might return to anything resembling normal.

Tim Cook, Apple’s chief executive, said on an earnings call this week that many of its suppliers’ factories in China would remain closed until Feb. 10.

Apple had closed one retail store in China, and said traffic into its stores across China had decreased in recent days. Apple is frequently “deep cleaning” its stores and regularly checking the temperature of its employees there, Cook said.

The airlines, meanwhile, are bracing for prolonged turmoil. American said all of its flights between the U.S. and mainland China were being suspended immediately, through March 27.

United and Delta said their flights on those routes would be suspended by next Thursday. United said it expected to resume operations March 28, while Delta said its suspension would last through April 30. The three airlines accounted for more than a third of all travel between China and the U.S. in 2018.

In 2018, more than 8.5 million passengers traveled between the U.S. and China, according to data from the U.S. Transportation Department. Most flew on a handful of Chinese airlines, none of which immediately responded to requests for comment Friday on any plans to halt or modify service.

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Xiao Wang, co-founder and CEO of Seattle-based Boundless, a technology company that helps families navigate the immigration process, said overlapping travel restrictions related to the coronavirus and national security are creating confusion and disruption.

“Everyone’s trying to figure out what’s going on,” he said.

For local employers with significant numbers of Chinese employees and growing businesses in China, the travel restrictions pose significant challenges. Amazon and Microsoft this week restricted travel for employees who regularly travel to China to work with suppliers and vendors, certify manufacturers and the like.

“I think there’s going to be a general delay from an operations perspective, for a number of important initiatives across these large companies,” he said.

The new coronavirus has already sickened more people than the SARS virus did in the eight-month outbreak of 2002 and 2003.

Some public-health and policy experts said the restrictions announced Friday, weeks after the virus was discovered in China, might not do as much as officials hoped in containing the contagion.

At this point, sharply curtailing air travel to and from China is more of an emotional or political reaction, said Dr. Michael Osterholm, an epidemiologist and director of the Center for Infectious Disease Research and Policy at the University of Minnesota.

“The cow’s already out of the barn,” he said, “and we’re now talking about shutting the barn door.”