The explosive spread of the coronavirus in Wuhan, China, was driven by people whose disease was never diagnosed, a new study has found, suggesting that people with symptoms so mild that they may not have known they were sick unwittingly spread disease in China and ultimately around the world.

Jeffrey Shaman, an infectious-disease expert at Columbia University who co-wrote the study, estimated that the true number of infections worldwide is larger than what has been reported. Given that there have been 169,000 confirmed cases in more than 150 countries, he said, more than 1 million people may be infected with the novel coronavirus globally.

The prevalence of “stealth transmission,” Shaman said, underscores the need for social distancing. When even healthy-seeming people can transmit the virus, the safest thing to do is to stay away.

Shaman’s study, published Monday in the journal Science, modeled the spread of the disease at its origin in Wuhan based on epidemiological research and mobile phone data.

Early in the epidemic, it suggests, there were six people with undocumented infections for every confirmed case of the disease. These cases were about half as infectious as the diagnosed ones, but because there were so many of them, they were behind at least two-thirds of documented infections, the researchers said.

After China imposed travel restrictions and ramped up testing on Jan. 23, the ratio of documented to undocumented cases flipped, Shaman said, so that about 60 percent of people carrying the virus were diagnosed. The spread of the virus in mainland China peaked a few weeks later. Now the number of new cases reported daily is down from thousands to tens.

“A radical increase in the identification and isolation of currently undocumented infections would be needed to fully control” the novel coronavirus, the study says.

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