WASHINGTON Reuters) - The Trump administration, while insisting the risk to Americans from coronavirus is low, nevertheless declared a public health emergency on Friday and announced the extraordinary step of barring entry to the United States of foreign nationals who have recently visited China.

People wear surgical masks in the subway station at Times Square in New York, U.S., January 31, 2020. REUTERS/Bryan R Smith

In addition, U.S. citizens who have traveled within the past two weeks to China’s Hubei Province - epicenter of the coronavirus epidemic - will be subject to a mandatory quarantine of 14 days, the incubation period of the virus, officials said.

Americans who visited other parts of mainland China will undergo special health screening upon their return, followed by up to 14 days of “monitored self-quarantine,” under the temporary restrictions.

The emergency measures were unveiled by U.S. Health and Human Services Secretary Alex Azar at a White House briefing, shortly before the Centers for Disease Control and Prevention (CDC) and local health authorities announced a seventh U.S. coronavirus case had been confirmed in Northern California.

The latest U.S. patient was identified only as a man in Santa Clara County, south of San Francisco, who became ill after traveling to China and has “self-isolated” at home, Sara Cody, director of public health for the county, told reporters.

She said the CDC was seeking to determine whether the man was infectious while flying home.

The U.S. entry ban on foreign travelers to China and the quarantine for Americans returning from Hubei go into effect on Sunday at 5 p.m. EST (2200 GMT), Azar said.

At the same time all commercial flights from China would be restricted to international U.S. airports in one of seven cities - New York, Chicago, Atlanta, Los Angeles, San Francisco, Seattle and Honolulu.

An average of more than 14,000 people traveled by air to the United States from China each day last year, either by direct or indirect flights, according to the White House.

KEEPING RISK LOW

CDC Director Robert Redfield told reporters the U.S. government acted after the World Health Organization declared a global health emergency on Thursday over the spread of the respiratory disease.

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“I want to emphasize that this is a serious health situation in China, but I want to emphasize that the risk to the American public is currently low,” Redfield said. “Our goal is to do all we can do to keep it that way.”

The U.S. State Department warned Americans on Thursday not to travel to China because of the epidemic.

The ban on U.S. entry of foreign nationals who have traveled to China during the past 14 days would exempt immediate family of U.S. citizens and permanent U.S. residents, Azar said.

The suspension-of-entry proclamation, signed by President Donald Trump, says the ban stays in effect until the president lifts it, and that the HHS secretary will recommend every 15 days whether to do so.

The flu-like coronavirus, which is believed to have originated in a seafood and animal market in Wuhan, the provincial capital of Hubei, and was first identified earlier this month, has resulted in 259 deaths in China.

More than 11,700 people have been infected in China, and more than 130 cases reported in at least 25 other countries and regions, with Russia, Britain, Sweden and Italy all reporting their first cases on Thursday or Friday.

None of the U.S. cases has been fatal, and all but one of the patients in the United States was believed to have contracted the disease while they were traveling in the Wuhan area of China.

FIRST U.S. QUARANTINES

The first quarantines of U.S. citizens potentially exposed to coronavirus in China began hours before the White House announcement.

Nearly 200 Americans evacuated earlier this week from Wuhan and voluntarily confined to a California military air base for 72 hours of health screenings were placed under a mandatory 14-day quarantine on Friday. It marked the CDC’s first quarantine order in 50 years.

The State Department said Friday it was working with Chinese agencies to arrange additional flights of Americans out of Wuhan. Washington also plans to evacuate non-essential government employees and family members from the U.S. Embassy in Beijing and consulates in Chengdu, Guangzhou, Shanghai and Shenyang due to the outbreak, a State Department official said on Thursday.

The two-week quarantine of the 195 Americans at March Air Reserve Base, near Los Angeles, runs from the time the evacuees left China on Tuesday.

The original plan was to release the passengers after 72 hours of evaluation and tests, absent any indication of illness, and permit them to take public transportation home. Local health authorities would then continue monitoring the evacuees through the remainder of the incubation period.

CDC officials said then that such a plan posed little or no risk of spreading the virus because individuals incubating the infection before symptoms appear are generally not contagious.

But Dr. Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases, told reporters on Friday that experts’ understanding of the virus was still evolving.

She cited emerging evidence the virus can be spread by someone who is infected but not yet showing signs of being ill, such as fever, cough and other respiratory symptoms.

The CDC also pointed to a limitation of its screening test for the virus - a negative result is merely a “point-in-time” snapshot that cannot conclusively rule out the risk of a person developing the disease during the 14-day incubation period.

The blanket quarantine at March air base was instituted after one passenger sought to leave the base without permission on Wednesday night, and was immediately slapped with an individual quarantine order by local health officials.

As of Thursday, none of the group at the base had exhibited signs of the disease, local health officials said.