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Yes, You Can Spread Coronavirus Even If You Don’t Have Symptoms.


There’s one question about the new coronavirus that keeps coming up over and over again: Why should I have to stay at home, avoid seeing my friends, and not let my kids play with other kids if we’re all fine and healthy? None of us have symptoms, none of us have been exposed, none of us are high-risk. We don’t even have many cases in our area. Why do we still have to isolate ourselves as much as possible?

The short answer: Because we know that the new coronavirus can spread before people have symptoms. And we know that not everyone is getting tested. So, it’s entirely possible that while everyone in your immediate circle seems fine and safe, there’s (at least) one person who is unknowingly spreading the virus without any symptoms at all. It’s also possible that this is happening outside your immediate circle, but somewhere else in your town, leading to a swath of cases that no one will see coming until some people do start to have symptoms, end up in the hospital, and start getting tested.

This is why we all need to be social distancing—even when it seems like it shouldn’t apply to you.

But what we’re still figuring out is when exactly the new coronavirus is contagious in people without symptoms, when you can expect to see symptoms after being exposed, and whether or not the people who never get symptoms can still actively spread the disease around.

To sort through what we know about these questions, I reached out to virologist Chad Petit, Ph.D., an assistant professor in the Department of Biochemistry and Molecular Genetics at the University of Alabama at Birmingham, and Caroline Colijn, Ph.D., an infectious disease modeler at Simon Fraser University in Canada.

This is how a virus spreads.

First, let’s discuss what we mean when we say “viral transmission,” which refers to the process by which viruses spread from host to host. “Typically, this is person to person but can also include viruses jumping from one species to another,” Petit tells SELF. (It’s thought that the new coronavirus entered the human population through this mechanism, probably from a bat but potentially via another animal species.)

The new coronavirus mostly seems to be spreading via “droplet transmission.” This happens when someone close to you (within about six feet) is coughing, sneezing, talking, or even just breathing and releases droplets containing the virus, which can then land on your nose and mouth and enter your system. These droplets can also land on surfaces, like countertops and doorknobs, which you might then touch. If you touch your face afterward, especially your eyes, mouth, or nose, the virus can get into your body.

Once the virus is in your body, it can attach to and enter your cells. While inside a host cell, “the virus shuts down the cell’s defense mechanisms and commandeers your cell’s resources to make more viruses, essentially turning your own cell into a virus-producing factory,” each of which can release more viruses to start the process all over again on other cells, Petit says.

ER doctor explains how they're handling COVID-19


This kicks off the incubation period, which is the timeframe when your body is producing more of the virus but you’re not yet showing symptoms like a fever, aches, coughing, and shortness of breath. “During this time, there may not be any clinical symptoms to alert the person that they are, in fact, infected,” Petit explains, but you could still be infecting other people. This is called “presymptomatic transmission,” and it’s the reason why you can’t just say, “Well, I’m not sick, and no one I’m spending time with is sick, and none of us have been exposed to anyone showing symptoms—so aren’t we in the clear?” Unfortunately, it doesn’t always work like that.

Research does back up the idea that people are likely spreading the new coronavirus before they have symptoms. A recent Centers for Disease Control and Prevention (CDC) report found that 331 of 712 people on the Diamond Princess cruise ship who tested positive for the SARS-CoV-2 virus that causes COVID-19 didn’t have any symptoms when they tested positive. That amounted to 46.5 percent of those with positive tests—so, almost half. They could have had the potential to spread the virus at that time, but we aren’t sure how much that actually happened. Another CDC article described how the new coronavirus spread from one teacher to two others during a dinner meeting on January 6. The first person in this trio to develop symptoms started feeling ill two days after the dinner, and the others—who aren’t known to have had any other potential COVID-19 exposures—developed symptoms four and six days after the dinner, respectively. They all appeared to be in fine health at the dinner, but it seems that one teacher still spread the new coronavirus to the others, who then reportedly spread it to some of their family members.

We still don’t know exactly when in the incubation period someone starts being contagious.

To determine the incubation period, we test sick individuals, figure out who they were in contact with and who might have been exposed to the virus, and then follow those exposed contacts over time to see if they get sick. If we know when those individuals were exposed and when they came down with the illness, then we can figure out the incubation period. “Currently, the incubation period typically lasts for 2 to 7 days ([with an] estimated median incubation period of 5.1 days), with 98 percent of those infected developing symptoms within 11.5 days,” Petit explains, adding that these numbers might shift when we have more information from new cases. A small percentage of people seem not to show symptoms until closer to 14 days.

But that still doesn’t tell us exactly when during the incubation period a person can spread the virus. It can’t be right at the beginning, because the virus hasn’t started to grow inside a person’s system at that point. Colijn is currently working on a project to figure this out.

“We can compare two things: (a) the incubation period,” she tells SELF, “and (b) the serial interval, the time between one person getting symptoms and someone they infected getting symptoms.” Colijn and her fellow researchers have described their findings in this study, which hasn’t yet been peer-reviewed but still offers interesting insights into this question. When analyzing information from Singapore and Tianjin, China, they found that the serial interval was shorter than the incubation period, meaning people seem able to spread the new coronavirus before they feel sick. Specifically, the study suggests that people may be able to transmit the virus to others at least three days before their own symptoms develop.

And what about people who test positive but never seem to develop symptoms?

The CDC study of cruise ship COVID-19 transmission determined that almost 18 percent of people on the Diamond Princess who had the new coronavirus never showed symptoms at all, then recovered. They apparently remained asymptomatic.

“Data on those who are completely asymptomatic is a major gap right now,” Colijn says. She explains that we can start to fill this gap when we can do large-scale serological tests to measure COVID-19 antibodies in people’s blood. This will tell us who was exposed to the virus at some point in the past but may never have known they were infected because they never felt ill. But until we have those widespread testing capabilities, we won’t have the full picture of who exactly is spreading the virus and when.

If people can spread the virus before they have symptoms, controlling the spread requires drastic measures.

For now, we’re still left with questions about how and when asymptomatic and presymptomatic COVID-19 spread is occurring. But we do know this: An outbreak is more difficult to control if we can spread the disease even when we don’t have symptoms. Yes, as Petit notes, it can seem completely counterintuitive that you could have COVID-19 but feel fine. But when people try to go about their daily lives as much as possible right now, it’s enhancing the spread of the virus, Petit explains. There’s really no way around that. This means that we can’t just take precautions if we’re feeling sick or think we’ve been exposed to someone who is sick.  

This is why social distancing is critical right now (along with other important practices like washing your hands well and often). It’s why we’re telling you not to meet up with your friends, or go out to the bar that’s still open in your town, or schedule a playdate for your kids, or otherwise carry on as you normally would. We can’t just assume that we would know if someone was transmitting the virus. Because, at this point, we don’t.

Even if you think your risk of getting sick is low, don’t let that perception make you feel like the rules don’t apply to you. You have no way of knowing if anyone you come into contact with has the virus or was recently exposed to someone who does, regardless of symptoms. And if you think you’re not at risk because you’re young and healthy, you should know that experts are starting to realize even younger people with no underlying health conditions can become extremely sick and, tragically, even die from COVID-19.

“We will be unlikely to control this by only isolating ill people,” Colijn says. “[There’s] need for broader measures—keeping away from each other—even if we don't know that we are sick.” She also cautions patience and notes that we won’t see the effects of social distancing immediately. “Cases that are confirmed today were infected some time ago, and we have only just started seriously physically distancing ourselves,” she says. “So take heart,” she adds. “We hope to see the results soon.”

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