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March 17, 2020 -- Unrecognized, milder cases of COVID-19 are likely the driving force behind the rapid spread of the virus throughout the world, a new study suggests.

The study, which was an international collaboration between researchers in China, Hong Kong, London, and the U.S., estimates that in the period before travel restrictions were announced in China, only 14% -- or about 1 out of 7 -- of infections were recognized.

People with recognized infections were sicker and more contagious. They are the ones who got and largely followed orders to isolate themselves from others.

A whopping 86% of infections were mild enough that people didn’t seek medical care or confirmation of their symptoms. These milder cases were only about half as contagious as the recognized infections, but because there were so many more of these undocumented cases than the documented ones, the unrecognized cases were the ones fanning the flames of the outbreak.

The study was published on Monday in the journal Science.

The study has profound implications for the spread of the disease within the U.S. Researchers in Seattle, who have been studying genomes of the virus isolated from people with known infections, have estimated that the virus has been quietly circulating here for about 8 weeks.

“If somebody is experiencing mild symptoms, and I think most of us can relate to this, we’re still going to go about our day,” said Jeffrey Shaman, PhD, a professor of environmental health sciences at Columbia University’s Mailman School of Public Health. “If we have a little bit of a headache or slight fever, we might take some ibuprofen and still go out shopping and whatnot. And it’s that sort of continued contact with people that allows the silent transmission of many respiratory viruses,” Shaman said in a press briefing Monday.

Based on the study’s findings, Shaman said the 150,000 confirmed infections worldwide probably represent just the tip of a much larger iceberg.

“We’re probably approaching close to a million infections globally,” he said. “Generally, you’re looking at about an order of magnitude more cases than have been confirmed.”

In the U.S., that means the 3,500 cases confirmed as of Monday by limited lab testing probably represent about 35,000 infections nationwide, said study author Elizabeth Halloran, MD, a member of the vaccine and infectious disease division at the Fred Hutchinson Cancer Research Center in Seattle.

“This is really the thing that underlies the whole reason why we need to increase social distancing,” Shaman said. “The virus thrives off the ability for people to mingle and meet each other.”

Show Sources

Jeffrey Shaman, PhD, professor of environmental health sciences, Columbia University’s Mailman School of Public Health, New York City.

Elizabeth Halloran, MD, DSc, Vaccine and Infectious Disease Division, Fred Hutchinson Cancer Research Center, Seattle.

Science: "Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV2)."

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