The World Health Organization on Tuesday raised the possibility that the new virus spreading in parts of China may be transmitting in an ongoing, sustained manner between people — which, if confirmed, would make it significantly more difficult to stop.

The agency’s Western Pacific Regional office, which covers China and neighboring countries, said on Twitter that new information “suggests there may now be sustained human-to-human transmission.”

“But more information and analysis,” it added, “are needed on this new virus to understand the full extent of human-to-human transmission and other important details.”

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The WHO’s defines sustained human-to-human transmission as transmitting easily from one person to the next and then further onward — in the way that flu or other established human viruses work. That’s in contrast to limited person-to-person transmission, in which a virus dies out after infecting a person or a few people in clusters of people who are in close contact with each other, such as in a family or a work setting.

International outbreaks of other diseases such as bird flu viruses or MERS — Middle Eastern respiratory syndrome — have been relegated to limited human-to-human transmission, not sustained transmission.

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The former form of transmission would be what one would expect to see in an animal virus that had not fully adapted to spread among people. The latter would signify a virus that is adept at infecting humans.

A spokesman for the WHO’s headquarters, in Geneva, told STAT in an email that it’s impossible at present to know whether sustained transmission is taking place.

“Current information does not allow for the full understanding of the extent of human-to-human transmission and the size of the epidemic,” Tarik Jasarevic wrote, saying that “scattered” human-to-human spread triggered by multiple animal-to-human transmissions may still be account for the majority of cases.

“We do not currently know which scenario is unfolding, but will continue to share our analysis,” he said.

The suggestion that sustained transmission may be happening will ratchet up already high concerns about the new virus, which is provisionally called 2019-nCoV. The virus is a coronavirus, from the same family as the viruses that caused SARS and MERS. While the source of the new virus is not yet known, Chinese authorities have said they believe it was transmitted to people from some type of wild game animal.

“Each day our new findings are not good,” said Michael Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota. “This is beginning to look more like SARS every hour.”

In late 2002, a new coronavirus started to spread in China, which for several months denied there was a problem. In February 2003, the virus began to spread broadly internationally, eventually infecting about 8,000 people and killing close to 800 before the outbreak was stopped.

As of Tuesday, the rapidly changing number of infections caused by the new virus was approaching 300, with at least six deaths reported. Most of the cases are in the central Chinese city of Wuhan, though cases have also been reported in Beijing and in Guangdong province, near Hong Kong. Thailand, Japan, and South Korea have also reported small numbers of cases in tourists from Wuhan or nationals who visited the city.

Tom Inglesby, director of the Johns Hopkins Center for Health Security, said if it is confirmed that there is sustained person-to-person spread of the virus, that will make stopping the outbreak a much more difficult task.

“That has very important implications for the response. It becomes much more challenging to control an outbreak with sustained human-to-human transmission, clearly,” he said, adding that stopping the virus from spreading in people should still be the goal.

The SARS outbreak was also a major challenge, he said, but the virus was driven out of the human population through strong public health interventions, and good international cooperation — which are needed now, Inglesby said.

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