The Centers for Disease Control and Prevention upgraded its response to the outbreak of a mysterious respiratory illness to a Level 3 Warning on Thursday.
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Meanwhile, the State Department is keeping its China travel advisory at a Level 2, advising travelers to "exercise increased caution."
As of Thursday, China decided to lock down three cities that are home to more than 18 million people in an unprecedented effort to try to contain a deadly new viral illness that has sickened hundreds and spread to other cities and countries in the Lunar New Year travel rush.
U.S. health officials said Wednesday they are actively monitoring 16 people who came into close contact with the traveler to China who became the first U.S. resident with a new and potentially deadly virus.
The man, identified as a Snohomish County, Washington, resident is in his 30s, was in good condition and wasn't considered a threat to the public. The hospitalized man had no symptoms when he arrived at the Seattle-Tacoma airport last week, but he started feeling ill. He had traveled to China in November, flying home to Washington state Jan. 15 before the start of U.S. airport screening.
Investigators will make daily phone calls to those 16 who had contact with him, including some who sat near him on his flight, to check if they have symptoms. They will not be asked to isolate themselves unless they start feeling ill.
Chinese state television puts the number of coronavirus cases at 634 as of Thursday morning, according to Reuters.
Shan Li, a reporter from The Wall Street Journal, took one of the last trains out of Wuhan to get to Beijing.
"Wuhan city officials have suspended outbound flights and trains and also public transportation within the city," she said in a video. "Panic had started to set in [at the railway station]. ... Tickets were being sold out. Police were starting to turn people away at the main entrance."
In addition, Chinese tech company Huawei is postponing a developer conference it had planned for mid-February in Shenzen, a city roughly 700 miles from Wuhan.
The Associated Press contributed to this report.