The latest evidence suggests it's more common than previously thought.

By Leah Groth
Updated May 29, 2020
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The advice from the US Centers for Disease Control and Prevention (CDC), World Health Organization, and other reputable agencies on how to protect yourself from coronavirus is simple: wash your hands frequently, wear a mask, maintain physical distance between yourself and others, and stay home if you have symptoms (except to get medical care). But now there's compelling evidence that even people without symptoms may be transmitting the virus.

Until recently, health officials were "just guessing" at the level of asymptomatic spread occurring in the population, according to Anthony Fauci, MD, director of the National Institute of Allergy and Infectious Diseases. During an April 5 White House Coronavirus Task Force media briefing, he put the number of asymptomatic cases "somewhere between 25 and 50%."

Nate Favini, MD, medical lead of Forward, told Health in March, that"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."

As CNN reported in May, the CDC has modeled the effects of asymptomatic spread for planning purposes. Based on current assumptions, the agency has estimated that 35% of coronavirus infections are asymptomatic and that 40% of transmissions are occurring before people develop symptoms.

What the latest research tells us about asymptomatic spread

Symptomless spread seems to be more prevalent than previously imagined. One report from China, published in JAMA Network Open, looked at a group of 78 people who tested positive for SAR-CoV-2, the virus that causes COVID-19. All were either were exposed to the Hunan seafood market (the site of early cases of the coronavirus) or had close contact with someone hospitalized with the illness. Of those individuals, 42% were asymptomatic. Compared to those who experienced symptoms, the asymptomatic group was younger and predominantly female. They also shed virus for a shorter period of time.

Separately, researchers in Australia, reporting in the journal Thorax, describe the results of COVID-19 testing of passengers and crew aboard a cruise ship that departed from Argentina in mid-March amid the global pandemic. All were screened for symptoms before boarding, and no passengers or crew from hard-hit countries were permitted to board. On day three, a decision was made to terminate the cruise. By day eight, one passenger had developed a fever, and then others fell ill. Ultimately, of the 217 people on board, 128 tested positive. Of those with confirmed COVID-19, only 24 had symptoms, while the majority—a stunning 81%—had no symptoms at all.

There were early warning signs of rapid transmission by asymptomatic carriers

An early instance of possible asymptomatic spread in the US took place during a Biogen conference at the Boston Marriott Long Wharf in late February. According to CNN, 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 March 27, the Massachusetts Department of Public Health had traced 99 cases of coronavirus in the state to Biogen conference attendees and their household contacts.

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 Diseases, 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 US, France, China and Hong Kong, explained in a press release. "That means we need to move quickly and aggressively to curb the emerging threat."

People appear to be spreading the virus before they know they're sick

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. 

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.

It's no wonder that William Haseltine, PhD, president of the global health think tank ACCESS Health International, recommends "contact tracing," which he says has already been implemented in Singapore and South Korea. This 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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