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Up to 25% of people with COVID-19 may not show symptoms

Residents wearing face masks wait outside a store in Codogno, southeast of Milan, Italy, on March 11, 2020, just a day after Italy imposed unprecedented national restrictions on its 60 million residents to help slow the spread of COVID-19.
Residents wearing face masks wait outside a store in Codogno, southeast of Milan, Italy, on March 11, 2020, just a day after Italy imposed unprecedented national restrictions on its 60 million residents to help slow the spread of COVID-19.
(Image: © MIGUEL MEDINA/AFP via Getty Images)

A significant portion of COVID-19 patients may never show coronavirus symptoms at all — a key factor aiding the spread of the virus, according to a top U.S. health official.

As many as 25% of people infected with the new coronavirus remain asymptomatic, Dr. Robert Redfield, director of the Centers for Disease Control and Prevention, told NPR this week.

"That's important, because now you have individuals that may not have any symptoms that can contribute to transmission," Redfield said.

What's more, even if people do get sick, they may shed the virus up to 48 hours before they show symptoms, Redfield added. "This helps explain how rapidly this virus continues to spread across the country, because we have asymptomatic transmitters and we have individuals who are transmitting 48 hours before they become symptomatic," Redfield said.

Recent studies from other countries also show that asymptomatic transmission of COVID-19 is likely occurring. For example, a study published today (April 1) in the CDC journal Morbidity and Mortality Weekly Report found that, of the more than 150 non-travel-related COVID-19 cases in Singapore, 6.4% appeared to involve asymptomatic transmission. In another study, published March 19 in the journal Emerging Infectious Diseases, scientists reviewed data on COVID-19 in China outside of Hubei province, and found that 12.6% of cases appeared to involve asymptomatic transmission.

In light of this data on asymptomatic spread, the CDC is "aggressively" reviewing its recommendations on the use of face masks and whether people should wear them if they don't have symptoms, Redfield said. Currently, the agency recommends face masks only for health care providers and people who are sick with COVID-19.

Still, there is a concern that such a recommendation could reduce the supply of face masks for health care workers who desperately need them. 

"The thing that has inhibited [a broader recommendation] a bit is to make sure we don't take away the supply of masks from the health care workers who need them," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told CNN. "But when we get in a situation where we have enough masks, I believe there will be some very serious consideration about … broadening this recommendation of using masks. We're not there yet, but I think we're close to coming to some determination." 

Originally published on Live Science. 

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