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Spread of the new coronavirus that originated in China has not yet reached a level that would deem it a global public health emergency, the World Health Organization said on Thursday. The virus has sickened more than 600 people, and 25 have died.

"Now is not the time. It's too early to consider that this event is a public health emergency of international concern," Didier Houssin, chair of the WHO emergency committee, said during a news conference from Geneva.

Houssin said the decision is based on the limited number of cases worldwide, as well as efforts in China to try to contain the disease.

"Make no mistake, this is an emergency in China," said WHO Director-General Dr. Tedros Adhanom Ghebreyesus. "But it is not yet a global health emergency."

Most of the patients have ties to the epicenter of the outbreak: Wuhan, China, though several cases have been reported in other Asian countries. One patient has been diagnosed in the U.S.: a resident of Washington state who had traveled to Wuhan and is now recovering in a hospital.

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Three cities in central China were put on lockdown in an effort to stop the spread of the respiratory illness. It's an extraordinary measure, particularly this week, as hundreds of millions of Chinese residents were expected to travel in advance of the Lunar New Year, a major holiday, on Saturday.

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Tedros said he hopes the travel restrictions will be "both effective and short in duration."

The WHO committee plans to come up with an official name for the illness caused by the new virus, now known as 2019-nCoV. That means it's a novel, or new, coronavirus that was discovered in 2019.

"For the time being, this name is OK. We can work with it, and it's understood by everyone on the planet," Dr. Sylvie Briand, director of the WHO's pandemic and epidemic diseases department, said during the press briefing, adding the committee members hadn't had time to discuss the name.

Naming an illness can be difficult and contentious. Public health officials generally avoid linking a virus to any particular region, especially one that can cause severe illness or death.

The WHO's official declaration of a "public health emergency of international concern" is reserved for unusual and serious public health events that have the potential to spread disease worldwide.

The designation has been used sparingly in recent years, including during the 2009 H1N1 (swine flu) pandemic; the 2014 Ebola outbreak in West Africa; and the Zika virus epidemic in 2015-16.

The hope is that such a formal declaration from the WHO would improve and streamline information gathering and sharing about the new illness about which little is known, as well as potentially increase funding for the response.

Earlier this month, Chinese researchers shared the full genomic sequence of the new virus to public databases, making it possible for health officials worldwide to study it and test for it.

Preliminary analyses suggest the new virus may share some genetic similarities with SARS, or severe acute respiratory syndrome. SARS also originated in China, and spread quickly to more than two dozen countries. More than 8,000 people became ill during the 2003 outbreak, and nearly 800 people died.

The new coronavirus is a different strain, and it's unknown whether it will be as severe or as contagious as SARS.

In a letter published Thursday in the Journal of the American Medical Association, researchers from National Institute of Allergy and Infectious Diseases wrote that "so far, it appears that the fatality rate of 2019-nCoV is lower than that of SARS-CoV."

But the outbreak is still evolving, and much of it remains unknown. There is no specific treatment for the new virus, though several antivirals and potential vaccines are under preliminary investigation.

"We have to be very, very careful in the beginning of an epidemic in making any pronouncements about the true severity," Dr. Michael Ryan, executive director of the WHO's health emergencies program, said at the news conference.

"It’s extremely important that we stick to the facts," Ryan said. "The facts are that 17 people have died. Their families grieve them this evening."

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