Silent Spreaders: The Dangers of Asymptomatic Coronavirus Carriers

"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.

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.