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How Does The Coronavirus Spread So Easily? Many People Don't Know They Have It09:47
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Experts say the coronavirus spreads so quickly because many people don't know they have it. (Timothy A. Clary/AFP/Getty Images)
Experts say the coronavirus spreads so quickly because many people don't know they have it. (Timothy A. Clary/AFP/Getty Images)

Scientists say one reason it’s so difficult to control the spread of the coronavirus is that many infected people only show mild symptoms — or no symptoms at all.

These “silent spreaders” don’t know they’re contagious, thus they further the spread by continuing their daily routines of work, school or activities.

Research published in the journal Science this month discovered 86% of all infections that happened before China instituted severe travel restrictions in late January were undocumented.

One of the researchers, Jeffrey Shaman, who's also a professor of environmental health sciences at Columbia University Mailman School of Public Health, says the silent spreaders they looked at had mild symptoms such as a sore throat, chills or body aches. However, these symptoms didn’t slow them down, he says.

“They may, for the most part, have experienced some symptoms at some point, but it didn't keep them home, didn't stop them from getting on public transportation, going to work, going to school, getting on airplanes and going on business trips,” he says.

Ultimately, he says, the researchers found the people with undocumented infections “contributed to the vast majority of the spread” of the coronavirus.

Interview Highlights

On silent spreaders being a major driver of advancing the spread of coronavirus

“What we found was that 86% actually of people were undocumented infections, meaning that they mostly fell in this category of mild symptomology and still going about their business. We also found that per person, these people were little more than half as contagious as the people who were admitted to hospitals or sought clinical care. So while that on a per person basis they were less contagious, because there's so many more of them, they actually contributed to the vast majority of the spread. We can actually take the model that we use to make these inferences and identify these numbers, and we can then shut off their ability of these undocumented infections to actually spread the disease at all. And what we find when we do this is that 80% of documented cases disappear as well.

“These silent spreaders are going out in the community and they're distributing the virus and they're setting up chains of transmission where a lot of people are subsequently becoming infected and they infect more people. The majority of those infections are, in fact, mild and are not people who then would then go and seek medical care. This is very consistent with what happens with a lot of the seasonal viruses that circulate around things like rhinovirus and influenza and the endemic coronaviruses. The majority of the infections are undocumented. People don't feel the need to go see a doctor. And as a consequence, we don't know that they're there. Those people don't stay home. They go about their business. They engage with the community pretty much as they normally would and they spread it broadly.”

On how to combat silent spreaders — and whether or not to test everyone

“[Testing everyone is] what some of the societies in the world have done. So what the Chinese did when they implemented these strong control measures on Jan. 23 is that they basically restricted travel, they shut down schools and businesses, they had everybody stay home, they enforced it forcibly and they did a lot of contact tracing. So they did a much more aggressive campaign of swabbing people to find out where the infections were, making sure those infected persons really stayed isolated and engaging in finding who their contacts were and quarantining them.

“The South Koreans did the same thing to very good effect when they had the outbreak in Daegu, which is their fourth largest city, and they were really informed by their prior experience with MERS, which is a related virus a few years back, and this actually proved to be effective, but the measures are very strong or somewhat draconian. The information that we hear is that the officials were going into people's credit cards, looking where they had shopped, looking where they had gone around, and then going to those locations and finding people who had been in contact with them potentially and testing them, a very aggressive campaign to identify where the virus may have spread.”

On determining the numbers within the research

“Ours is one study of this. The way we did it is that we're modelers. We make a mathematical model that describes the transmission of the virus in a way that we think represents reality. In this case, we represented the transmission within and between 375 cities in China. We informed that model with observations and movement data, and then we tried to run what's called an inference problem and estimate those critical parameters here. What fraction of the people are undocumented? What fraction infections are undocumented? And how contagious are they? The reality is it's consistent with what we see in terms of the spread of the virus and how quickly it moved geographically, both within China and outside of China.

“Now there is the added wrinkle that there is pre-symptomatic shedding of this virus, but our model actually does implicitly capture that. We've also applied it to the United States in a slightly different form. And here where testing is less, we're finding actually only one in 11 people are being documented. The other thing is after the travel restrictions are imposed, our same model estimates that about two-thirds of people were identified in China when they were in this period of much more active contact tracing and using of tests.”

On what the U.S. could do better in handling the virus

“Well, I do think there are some things we could do better. Look, the main issue here is that we're dealing with a very challenging virus that unfortunately is very dangerous for the world at large. And it is so because it has three ingredients that come together to make it so. Firstly, it's an emergent novel respiratory virus to which the majority of the population, if almost 100%, are susceptible to it. So those susceptible people, which is you and me, are the tinder that allows us to spread like wildfire. The fact that it's respiratory also allows it to transmit very efficiently. People don't necessarily have to overlap in time and space in order to have transmission, because somebody can walk into a room and deposit the virus on surfaces in the air and then somebody can come in later and actually pick it up from them.

“The second thing is that it has these mild infections that support the stealth transmission that the majority of people who are getting infected are not documented, and they're spreading it in the community unwittingly.

“And the third thing that makes it very dangerous is it has this fat tail where the symptom output is in some instances, the symptoms for people, are very severe and it's causing high rates of mortality, which is incredibly problematic. In order to deal with it, you really want to get out in front of it as quickly as possible. Testing is a really effective way of doing it, particularly early in an outbreak. It doesn’t mean you want to do it willy nilly, but it means that when you identify people who are infected, you want to find out who they were in contact with [and] quarantine those people. If they begin to manifest, test them and then test their contacts as well. The difficulty is that in a place such as New York right now where I live, it's gotten kind of out of hand. And the question is, is there the capacity, are there even the swabs available, to conduct testing at the level we would want to try to flatten that curve and bring it back down? And that's the challenge that we're facing right now.”

On how the virus is transmitted

“The answer, unfortunately, is that we don't know. One of the outstanding questions and issues associated with all respiratory viruses is that we really don't know the mode of transmission. That is the way in which they are transmitted and which one is dominant. We know how they can be. We know they can be airborne. We know [they] can be droplets. People spraying each other in the face when they cough or sneeze or speak or even breathe. We also know that it can be that some of those droplets settle on surfaces, that somebody else comes along and touches and then they eat a cheeseburger or they rub their eyes or whatnot and they're going to transfer it to themselves that way. Those are the three main routes that we're talking about for viruses like this. We don't have good evidence to even define how flu is transmitted. This is something that's hotly debated and has important implications for the control of the virus. We don't know the answer to that for this. So the question for the grocery store is to be careful, to wash your hands afterward, wash your produce and try not to get too close to people as you're moving around when you're having to expose yourself in situations like that.”


Chris Bentley produced and edited this interview for broadcast with Peter O'DowdSerena McMahon adapted it for the web.

This segment aired on March 24, 2020.

Jeremy Hobson Twitter Co-Host, Here & Now
Before coming to WBUR to co-host Here & Now, Jeremy Hobson hosted the Marketplace Morning Report, a daily business news program with an audience of more than six million.

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