On Friday, the Chicago Department of Public Health confirmed a second U.S. case of coronavirus, one day after the U.S. State Department issued its most severe travel advisory – Level 4: do not travel – for the central Chinese province of Hubei, home to Wuhan, the epicenter of the coronavirus outbreak.

Although the country as a whole maintains its Level 2 advisory (exercise increased caution), the State Department issued a special warning for the Wuhan region and noted that it had evacuated all non-essential personnel from China, limiting its ability to aid U.S. citizens in Hubei province.

"There is an ongoing outbreak of pneumonia first identified in Wuhan, China, caused by a novel (new) coronavirus. In an effort to contain the novel coronavirus, the Chinese authorities have suspended air and rail travel in the area around Wuhan," the advisory reads.

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The advisory continued, "Chinese authorities have imposed strict travel restrictions in the area around Wuhan. Travelers should be aware that the Chinese government could prevent them from entering or exiting parts of Hubei province. Travelers should be prepared for travel restrictions to be put into effect with little or no advance notice."

Wuhan is located about 715 miles (1,152 kilometers) south of Beijing. Getting there involves a nearly 5-hour flight or bullet-train ride.

Chinese officials have shut down train, subway, bus and ferry service in Wuhan as well as the neighboring cities of Huanggang and Ezhou. The measures have effectively quarantined 20 million people.

The State Department advisory also warned any American who has been to Wuhan in the last two weeks and has displayed symptoms of the virus, including fever, cough or respiratory problems to call ahead to their doctor or local emergency room to apprise them of their situation and then seek medical attention immediately.

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Second U.S. case confirmed in Chicago

Officials are not releasing the airline the Chicago patient traveled on from China but officials at the Centers for Disease Control and Prevention have been notified and will determine whether other passengers on the flight need to be contacted.

Dr. Allison Arwady, commissioner of the Chicago Department of Public Health and a former CDC official, said the woman did not have any symptoms when she was traveling so was likely not contagious on the flight.

“Our concern for transmission (to other people) before symptoms develop is low,’’ she said, adding that the information is based on what they know about how other coronaviruses, including the common cold, spread.

The woman arrived in Chicago on Jan. 13, before screenings began at O’Hare International Airport.

Those screenings began on Wednesday, according to the Chicago Department of Aviation.

O'Hare is one of five U.S. airports where the CDC is doing screenings on passengers traveling from Wuhan.

The CDC said Friday that 2,000 passengers arriving on 200 flights had been screened as of Thursday at the participating airports, with no confirmed cases.

But with a travel ban in Wuhan – airlines canceled more than 250 flights to and from Wuhan Tianhe International Airport on Friday, according to flight tracker FlightAware – and the two U.S. patients not showing symptoms until a few days after travel, officials are evaluating how long the enhanced screening will continue and whether it might be extended to cover travelers arriving from other airports in the region.

"As you would expect we are reevaluating the approach,'' Dr. Martin Cetron,director of the CDC's Division of Global Migration and Quarantine said Friday.

At least 63 potential cases are under investigation in 22 states, and 11 were confirmed negative, Messonnier said. The other potential cases are being tested, said Nancy Messonnier, the director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention.

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Senator asks Trump administration to consider travel restrictions

On Friday, Sen. Josh Hawley, a Missouri Republican, confirmed via tweet that he had written to four cabinet secretaries "to ask whether temporary travel restrictions from affected areas in China may be necessary, and if they are, when American travelers will be notified. Public safety must be #1 priority."

In his letter, addressed to Secretary of State Mike Pompeo, Transportation Secretary Elaine Chao, Acting Homeland Security Secretary Chad Wolf and Health and Human Services Secretary Alex Azar, Hawley wrote, "I appreciate that CDC has already initiated screening for the virus at major airports but I would urge you to expand this program as aggressively as possible. I also appreciate that CDC has issued a travel advisory for the region. However, I believe Americans deserve notice of your plans to evaluate more aggressive limitations on travel between the U.S. and China."

His letter, published by Scott Wong, a Congressional reporter for The Hill, also asked whether the federal government has a plan in place for making travel restrictions in order to rapidly respond to outbreaks such as the coronavirus epidemic.

U.S. airlines issue travel waivers

American, Delta and United have all issued travel waivers for travel to Wuhan. None of the carriers fly nonstop to Wuhan, but they offer connecting service, with their Asian carrier partners operating the intra-China flights.

The travel dates and destinations covered by the waivers vary by airline.

American has two waivers, one covering Wuhan and one covering Shanghai and Beijing, China, two major destinations. For Wuhan trips scheduled through March 31, travelers can request a refund even if they are holding nonrefundable tickets. For Beijing and Shanghai, travelers with tickets through Jan. 31 can change or postpone their trip without the usual penalties.

United's waiver covers travelers with tickets through March 29 and allows refunds.

Delta on Friday issued a waiver that covers two major destinations in China, Shanghai and Beijing. Travelers booked on flights to or from those cities through Jan. 31 can change their flights to as late as February without the usual penalties. Travelers who want to cancel their trip are eligible for a voucher in the amount of their ticket, though a change fee will apply when using it.

American, Delta and United each offer nonstop flights to Shanghai and Beijing from the U.S.

American responds to flight attendants' request for more safeguards

Late Friday, American said it plans to allow flight attendants to wear face surgical masks on Hong Kong, Beijing and Shanghai flights.

The CDC isn't recommending that airline crews wear masks, but American says many of its crew members are concerned and asked about the mask policy, spokesman Curtis Blessing said.

"As a result, we have adjusted our policy on a short-term basis as the situation evolves,'' he said in a statement.

On Thursday, the airline said it was providing additional hand-sanitizing wipes to flight attendants working all American flights between the U.S. and Shanghai, Beijing and Hong Kong.

American's face mask policy comes after the union representing its flight attendants said American and other carriers need to step up their precautions against coronavirus, despite Thursday's decision by the World Health Organization not to declare the outbreak a global health crisis or ask the airline industry to expand their efforts against the virus, which the WHO has officially named 2019-nCoV.

In a Thursday press release, the Association of Professional Flight Attendants asked American Airlines to "institute immediate emergency measures, including providing crew members the latest information regarding the 2019-nCoV outbreak, identifying the signs and symptoms of illness in oneself and others, and practical procedures to manage potentially ill persons."

Union president Lori Bassani said, “The health of our crew members and passengers is a top priority for us and we refuse to compromise their health or safety in any way. I am urging American Airlines and all airlines to do everything humanly possible to contain the outbreak and minimize any chance of exposure. We will continue to speak out to ensure airlines are erring on the side of caution and putting our members’ health first in these dangerous times.”

American's president, Robert Isom, said on the airline's earnings conference call Thursday that the airline is working closely with the CDC, U.S. Customs and Border Protection and public health officials to make sure it is following "best practices'' when it comes to public health issues."

"We're doing that with an intent to make sure that we take care of our customers and team members,'' he said.

There have been incidents this week at Los Angeles International, Chicago O'Hare and Boston's Logan Airport but no new confirmed cases of coronavirus, which The Who is now referring to as novel coronavirus. The only confirmed U.S. case is a Washington State resident who returned from Wuhan on Jan. 15. The Centers for Disease Control is tracking at least 16 individuals who have been in close contact with him.

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Contributing: Ryan Miller, USA TODAY