DALIAN, China/NEW YORK -- American health authorities on Tuesday confirmed the first case of a traveler in the U.S. infected with a new coronavirus from China, after Chinese officials reported that more than 300 people have been diagnosed so far.
America's Centers for Disease Control and Prevention said that the infected patient was in Washington state after returning to the country last week from the Chinese city of Wuhan, believed to be the source of the outbreak.
"This is an evolving situation and again we do expect additional cases in the United States and globally," Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases, said on a media briefing call.
Six deaths have been linked to the new virus, which causes pneumonia, as health experts assess whether it is becoming more infectious as it spreads.
"It is now very clear from the latest information that there is at least some human-to-human transmission" of the virus, the World Health Organization's Western Pacific office tweeted, citing infections among health care workers.
The CDC announced enhanced health screenings at two more airports, Hartsfield-Jackson Atlanta and Chicago O'Hare, for travelers arriving on direct or connecting flights from Wuhan. Screenings at New York's John F. Kennedy and San Francisco and Los Angeles airports began late last week.
Health authorities in Wuhan stated Tuesday that 15 medical personnel had contracted the virus.
Exactly how readily the virus spreads is an unanswered but critical question with the Lunar New Year travel season underway. Roughly 3 billion trips are expected to be taken over the 40-day period, many of them overseas.
The bulk of the Chinese cases -- 270 -- are in Hubei Province, where Wuhan is located. But infections have also been confirmed in Beijing, Chongqing, Shanghai and Tianjin, as well as in Guangdong, Henan and Zhejiang provinces.
John Wiesman, secretary of health for Washington state, said that at this stage the risk posed to the American general public by the new coronavirus was "low."
Cases have also emerged in Thailand, Japan and South Korea, and Philippine authorities are investigating a possible case. A 55-year-old Taiwanese woman was reported Tuesday to have tested positive for the virus.
The outbreak may be more widespread than official Chinese information suggests. A team of scientists at the University of Hong Kong estimated Tuesday that more than 1,300 people are likely to have contracted the virus in Wuhan alone.
New cases suggest "there may now be sustained human-to-human transmission," the WHO office tweeted, referring to transmission among people who have had no contact with the original source.
"We need to pay attention not only to persistence, but also to the reproductive number," or how many cases arise from a single infected patient, said virology professor Hitoshi Oshitani of Japan's Tohoku University.
Severe acute respiratory syndrome, or SARS -- also caused by a coronavirus -- was both highly persistent and had a high reproductive number. It infected more than 8,000 people between 2002 and 2003.
Middle East respiratory syndrome, discovered in 2012, did not have a high persistence but showed a remarkably strong ability to spread in certain environments, such as hospital dialysis units.
"In terms of human-to-human transmission, we need to consider the risk of a MERS-level outbreak," Oshitani said.