It's still important to steer clear of those showing symptoms, but new information shows that may not be enough.

By Leah Groth
March 17, 2020

The advice from the Centers for Disease Control and Prevention, World Health Organization, and other reputable agencies on how to protect yourself from coronavirus is simple: wash your hands frequently, don't share personal items, and try to keep your distance from those who are showing symptoms. But now, new research and anecdotal evidence shows that even those without symptoms may be able to transmit the coronavirus.

While it's still unclear exactly how much of the current coronavirus outbreak has been fueled by asymptomatic, mildly symptomatic, or pre-symptomatic people, the risk is there, Nate Favini, MD, medical lead of Forward, tells Health. That's because people who are asymptomatic can still spread the virus and have high levels of the virus in their respiratory secretions, he says—something that is apparent from the rapid spread of the coronavirus. "Given the rapidity of the spread of COVID-19, it seems plausible that people who are asymptomatic or mildly symptomatic could be playing a role in spreading the virus," he says.

Deborah Birx, MD, an American family physician who's currently serving as the response coordinator for the White House Coronavirus Task Force, has also voiced concern about potential asymptomatic carriers of the coronavirus. During a White House press conference Saturday, Dr. Birx mentioned that experts are still trying to understand people—mainly those under the age of 20—who don't have "significant symptoms": "Until you really understand how many are asymptomatic and asymptomatically passing the virus on, we think it's better for the entire American public to know that the risk of serious illness may be low, but they could be potentially spreading the virus to others," she said.

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One instance of this possible asymptomatic spreading took place in Massachusetts during a recent Biogen conference at the Boston Marriott Long Wharf. According to CNN, the conference too place in February, and after the meeting was over, three employees tested positive for coronavirus—but a spokesperson for the Massachusetts Department of Public Health, via CNN, noted that those three employees didn't show symptoms during the meeting. As of Monday, 108 cases of Massachusetts' total 164 coronavirus cases have been linked back to the Biogen conference.

New research from various outlets also points to asymptomatic or mildly symptomatic carriers, which may count for a significant amount of transmission. One research letter, set to be published in the CDC journal Emerging Infectious Disease, found that that time between cases in a chain of transmission is less than a week, with more than 10% of patients being infected by somebody who has the virus but does not yet have symptoms. "The data suggest that this coronavirus may spread like the flu," Lauren Ancel Meyers, a professor of integrative biology at UT Austin, who was part of a team of scientists from the United States, France, China and Hong Kong, explained in a press release. "That means we need to move quickly and aggressively to curb the emerging threat."

Two more studies—neither peer-reviewed, but posted on MedRxiv, a pre-print server founded by Yale University, the medical journal BMJ and Cold Spring Harbor Laboratory in New York—found that pre-symptomatic people were responsible for an overwhelming amount of overall spread.

One of the studies, shared by Dutch and Belgian researchers, analyzed two coronavirus clusters in Singapore and Tianjin, China, and found that between 48% and 66% of the 91 people in the Singapore region contracted the infection from someone who was pre-symptomatic, while between 62% and 77% of the 135 people caught coronavirus from someone who had yet to show symptoms in the Tianjin cluster. The other study, shared by Canadian, Dutch, and Singaporean researchers and also looked at the Singapore and Tianjin clusters, found that the coronavirus was transmitted, on average, 2.55 days before symptom onset in Singapore and 2.89 days before symptoms onset in Tianjin. 

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And in February, researchers in China published a research letter in the Journal of the American Medical Association, outlining a case of an asymptomatic woman in Wuhan, China who reportedly spread the virus to five family members while traveling to Anyang, China—all of whom developed COVID-19 pneumonia. "The sequence of events suggests that the coronavirus may have been transmitted by the asymptomatic carrier," wrote the study authors.

Despite the most recent information and research, US agencies, including the CDC, maintain that the coronavirus is mainly spread by symptomatic people—but according to William Haseltine, PhD, president of the global health think tank ACCESS Health International, it's this way of thinking that may render the current mode of testing for the coronavirus in the US as "dramatically ineffective."

Instead, Haseltine recommends a testing system known as "contact tracing," which, he says, has already been implemented in Singapore and South Korea. The method involves testing everyone with symptoms first; then, after identifying those with the virus, attempting to find and test every person the infected individual has come into contact with over a period of two weeks. Essentially, says Haseltine, it's not necessarily about how many tests an area has, but how they're used. "You want to catch people early, before they get sick" he says.

The information in this story is accurate as of press time. However, as the situation surrounding COVID-19 continues to evolve, it's possible that some data have changed since publication. While Health is trying to keep our stories as up-to-date as possible, we also encourage readers to stay informed on news and recommendations for their own communities by using the CDCWHO, and their local public health department as resources.

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