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Silent Spreaders: The Dangers of Asymptomatic Coronavirus Carriers

Some people are harboring and transmitting the virus without knowing it

woman on the street wearing mask looking wary of other pedestrians

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En español | Since the start of the pandemic, millions of people around the world have become sick — some critically so — with COVID-19, the disease caused by the new coronavirus. But not everyone with a coronavirus infection develops symptoms and falls ill. Researchers are learning that a significant portion of people who test positive for the virus never exhibit the warning signs.

"I think that's one of the reasons why transmission of this virus has been so hard to control and contain,” says Michael Mina, M.D., assistant professor of epidemiology at the Harvard T. H. Chan School of Public Health.


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If you don't know you have the virus and otherwise feel healthy, you're less likely to change your behavior “in a way that's commensurate with reducing transmission.” Mina says this creates a massive problem when it comes to controlling the outbreak, and is one reason why “this virus will continue to transmit.”

Contagious without coughing

People who have coronavirus infections but do not show symptoms fall into two categories: There are presymptomatic individuals, who don't have symptoms at the time they are tested but go on to develop them a few days later; and asymptomatic individuals, who remain symptomless throughout the course of the infection. And emerging data show that a lot more people than previously thought fit into these two groups.

A population-based study in Iceland, for example, found that 43 percent of participants who tested positive for the virus reported no symptoms at the time of the test. Similarly, about 45 percent of positive cases in an Indiana-based population survey were asymptomatic when tested, and about 18 percent of people onboard the Diamond Princess cruise ship never developed symptoms of a coronavirus infection.

"Now that doesn't prove that they were transmitting” the virus just because they had it, says Charles E. Davis, M.D., professor emeritus of pathology and medicine at the University of California, San Diego, and director emeritus of microbiology at the UCSD Medical Center. “But there's no question in my mind whatsoever that there is transmission from the people who have been asymptomatic.” Pinpointing just how big of a role people without symptoms play in the pandemic, however, is difficult to do.

"Keep in mind that just because somebody is not coughing, it doesn't mean that they're not a potential transmitter.”

— Anne Monroe, M.D., associate research professor of epidemiology at the George Washington University Milken Institute School of Public Health

"We're still not exactly sure what proportion of transmission is attributable to people who are asymptomatic,” says Anne Monroe, M.D., associate research professor of epidemiology at the George Washington University Milken Institute School of Public Health. Some experts believe that people are the most contagious when they are symptomatic because “they are more likely to cough, produce respiratory droplets, and they may have more infectious characteristics,” Monroe explains.

However, one doesn't need to cough to spread the coronavirus. It can be transmitted through talking, sneezing, exhaling and other everyday actions. And some studies show that people without symptoms of the virus “are presumably as contagious as people who have symptoms,” says Gigi Kwik Gronvall, an immunologist and senior scholar at the Johns Hopkins Bloomberg School of Public Health's Center for Health Security.

Those who are presymptomatic can also spread the virus before symptoms set in. This is why the Centers for Disease Control and Prevention (CDC) recommends that everyone, even those who feel healthy, wear a cloth face covering in public settings where other social distancing measures are difficult to maintain, for example, grocery stores and pharmacies. The primary role of a cloth face covering is to reduce the risk of the wearer spreading the virus to others, more so than protecting the wearer from infection.

Testing, tracing and tracking symptoms

Testing people on a wider scale — not just when someone shows symptoms — is essential to understanding how many coronavirus cases can be chalked up to unwitting silent spreaders, Monroe says. It's also key to slowing the outbreak because it helps to identify those who need to self-isolate before they spread the virus to others, she adds.


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Harvard's Mina says new contact tracing technologies that alert people if they were in the same vicinity as someone with the virus, for example, can also help catch asymptomatic carriers and contain their impact on the outbreak. It also may be the case that, as health care providers and researchers learn more about the virus, they will be able to identify additional signs and symptoms that can clue people in on whether they are infected, Gronvall says.

The CDC's initial list of coronavirus symptoms was limited to three: fever, cough and shortness of breath. That, however, has recently expanded to include everything from sore throat, to chills, to loss of taste and smell. Researchers have also identified a handful of unusual indications of the illness, including skin rashes and gastrointestinal (GI) issues. “And I'm sure that there'll be some other things” to add that will help people without the hallmark characteristics of the coronavirus better identify an infection, Gronvall says.

But even a longer list of symptoms will not cast a wide enough net to catch every coronavirus case. Similar to other viruses including herpes, influenza and norovirus, some people will pass on the coronavirus without experiencing a single symptom. CDC Director Robert Redfield, M.D., has said that as many as 25 percent of people with coronavirus infections could remain asymptomatic.

Assume everyone has the virus

As communities across the country begin to resume some sense of normalcy, precautionary measures, including temperature checks, are being worked into reopening plans. And while screening for symptoms such as fever will detect some people with the virus, it will not be sufficient in catching everyone, UCSD's Davis says.

That's why it's important to keep up with preventative measures until a vaccine is developed and distributed, Gronvall argues. “People don't flash a siren when they start becoming contagious. You have to be vigilant.”

Monroe's advice is to act as though everyone you come into contact with has the virus. Wearing a face covering, washing your hands often and keeping a distance of at least 6 feet between yourself and others “can help reduce transmission across the board, including transmission from people who are asymptomatic,” she explains.

"Keep in mind that just because somebody is not coughing, it doesn't mean that they're not a potential transmitter,” Monroe adds.

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