None of the 195 passengers who returned to the U.S. on Wednesday after escaping the coronavirus outbreak in China have shown signs of infection, but they will remain under observation for up to three days as they get screened, a CDC official said.

Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases for the Centers for Disease Control and Prevention, said all passengers voluntarily agreed to remain at March Air Reserve Base in Riverside, California, for monitoring.

"We have 195 travelers who are willingly undergoing isolation for the purpose of medical evaluation and investigation of their risk,’’ Messonnier said, adding that the process may take less than 72 hours.

The number of confirmed infections across China has risen to 7,711, resulting in 170 deaths. By comparison, there were 5,327 cases of severe acute respiratory syndrome (SARS) in China during the 2002-2003 outbreak, although that virus was more deadly, claiming 349 lives.

The American passengers flew into California from Wuhan, the city in central China at the epicenter of the outbreak, with a stopover in Anchorage, Alaska, where they had also been screened. The CDC revised the original number of travelers it said were on the flight down from 201. The group burst into cheers when initially touching down on American soil in Alaska.

Also Wednesday, Tedros Adhanom Ghebreyesus, director-general of the World Health Organization, announced that he will reconvene the International Health Regulations Emergency Committee on Thursday to advise him on whether the outbreak constitutes a public health emergency of international concern, or PHEIC. Last week, the committee found it was too soon to make such a declaration.

The committee's decision may be influenced by a new report from Chinese researchers suggesting person-to-person spread of the virus among close contacts occurred as early as mid-December. Based on the first 425 confirmed cases, the researchers estimate that each infection led to 2.2 others on average. That’s a bit more than ordinary for the flu.

Messonnier said the number of confirmed cases in the U.S. remains steady at five, out of 165 persons under investigation. Of those, 68 have tested negative and the rest are awaiting results. Symptoms include fever, cough and respiratory illness.

Though she expects more positive results, especially among those who have recently visited Wuhan or been in close contact with those travelers, Messonnier said people in the U.S. don't need to be especially concerned.

“At this time, we continue to believe that the immediate health risks from this new virus in the general American public is low,’’ she said.

Those most vulnerable to severe illness from the coronavirus have usually been weakened by existing conditions.

Researchers in China found 50 of 99 patients admitted to Wuhan hospitals with the virus from Jan. 1 to Jan. 20 had underlying chronic health problems, including cardiovascular disease and diabetes, according to a new analysis published in The Lancet.

Most of the patients were middle-aged men, and 46 of the patients worked at the Huanan seafood market, clinical records, lab results and epidemiological data showed. All of those hospitalized had pneumonia, most had a fever and cough and about one-third experienced shortness of breath. Five critically ill patients also suffered from bacterial fungi infections, the study found.

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The flight was chartered by the State Department to return a few dozen Wuhan consulate workers to the USA. Most of the passengers were Americans living in Wuhan who paid their way to get out.

The State Department said it was working with Chinese officials to evacuate other Americans in and around Wuhan who want to leave.

Secretary of State Mike Pompeo spoke by phone with Yang Jiechi, foreign affairs director for China's Communist Party. Pompeo expressed condolences for the loss of life and thanked Yang for his help in the evacuation.

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Wuhan, a city of 11 million in the central China province of Hubei, is one of more than a dozen under tight lockdown as the government struggles to contain the virus.

The British government announced plans to bring about 200 citizens back home from Wuhan on Thursday, then quarantine them for two weeks. France, which has four confirmed cases of the virus, also intends to quarantine citizens returning from Wuhan.

Michael Ryan, who heads the WHO health emergencies program, said he visited China and was amazed at its effort to combat the outbreak.

"The challenge is great, but the response has been massive," he said at a news conference in Geneva. "I am very impressed with the level of engagement of the Chinese government at all levels."

Ryan said about 20% of the confirmed cases involved serious illness and about 2% resulted in death, compared to 10% for the SARS coronavirus in China from 2002-2003.

Ryan said concern over the handful of cases spread from human to human outside China is one reason why a PHEIC is being considered.

Such declarations can mean more resources are available to combat outbreaks, but they can spur restrictions on trade and travel.

Coronaviruses get their name from their appearance under a microscope – they look something like a crown, a sphere with spikes jutting out.

A global scientific hunt for a vaccine is underway, but testing will take time.

Contributing: Jennifer Portman, USA TODAY; The Associated Press