Asymptomatic Vs Presymptomatic: How The Virus Spreads : Short Wave It's called asymptomatic spread. Recently a scientist with the World Health Organization created confusion when she seemed to suggest it was "very rare." It's not, as the WHO attempted to clarify.

NPR science reporter Pien Huang explains what scientists know about asymptomatic spread, and what might have caused the WHO's mixed messages.

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How Many People Transmit The Coronavirus Without Ever Feeling Sick?

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How Many People Transmit The Coronavirus Without Ever Feeling Sick?

How Many People Transmit The Coronavirus Without Ever Feeling Sick?

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  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
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Hey, y'all - quick note - here on SHORT WAVE, we will continue to regularly cover the pandemic. We've got another episode today - because even though it might feel like things are going back to normal, we are still right in the middle of this thing. Rest assured, we will still bring you stuff about space and dinosaurs and dinosaurs in space as well. And if you haven't subscribed or followed, now is the perfect time. OK, on to the show.


SOFIA: You're listening to SHORT WAVE from NPR.

Maddie Sofia here with NPR science reporter Pien Huang. Pien, how the heck are you?

PIEN HUANG, BYLINE: Maddie, I am doing pretty well right now.

SOFIA: So Pien, the story that you've brought us today starts last week.


UNIDENTIFIED PERSON: Hello, everyone, and welcome to this regular COVID-19 press briefing.

SOFIA: Scientists at the World Health Organization were having a virtual press conference, a normal thing they do, just giving updates on the virus and how it's moving around the world. And then something kind of confusing happens.

HUANG: Yeah. So Maria Van Kerkhove, who's one of WHO's top epidemiologists...


MARIA VAN KERKHOVE: So thanks for that question - absolutely.

HUANG: She was answering a question about asymptomatic transmission - about people who spread the virus to others while having no symptoms themselves. She talked about what the data looked like in a couple different settings, and then she said this.


VAN KERKHOVE: We are constantly looking at this data, and we're trying to get more information from countries to truly answer this question. It still appears to be rare that an asymptomatic individual actually transmits onward.

HUANG: Now, she seemed to be saying that if you have the virus but you never get symptoms from it, it's very rare that that person would pass the virus to someone else.


VAN KERKHOVE: It's very rare. And that - much of that is not published in the literature.

SOFIA: And that was confusing to people - right? - because a big reason of why we wear masks and social distance is that we don't know who could potentially have or spread the virus.

HUANG: Right. And listening to Van Kerkhove, you'd think - well, why did we lock down? Why do we wear masks? She left listeners with the impression that anyone without symptoms had a low chance of spreading the virus. And that's wrong. We know - and there's documented evidence for this - that the virus can be spread by people without symptoms.

SOFIA: So this statement was reported on by news outlets. You know, the WHO is a major global health organization. What they say matters. And that scientist, Maria Van Kerkhove had to kind of walk this back.

HUANG: Yeah. There was a lot of reaction and pressure from researchers and the public. People were saying that it was confusing at best or actually wrong at worst. So a day after she made that first statement...


VAN KERKHOVE: So there were quite a lot of messages that I received overnight...

HUANG: ...Maria Van Kerkhove did a Q&A on social media trying to explain.


VAN KERKHOVE: I was responding to a question at the press conference. I wasn't stating a policy of WHO or anything like that. I was just trying to articulate what we know. And in that, I used the phrase very rare. And I think that that's misunderstanding to state that asymptomatic transmission globally is very rare.

SOFIA: Pien, I believe that's what you call walking it back.

HUANG: I would completely agree with you.

SOFIA: OK. So Pien, today's episode, we're going to explain what might have happened here and talk through what we do know about how the virus spreads. You're listening to SHORT WAVE, the daily science podcast from NPR.


SOFIA: OK, Pien - as best we can guess, this mix-up with the WHO was partly about a distinction between asymptomatic people and what scientists are calling pre-symptomatic people.

HUANG: Yeah. So here's the difference. Asymptomatic, which is the group that Van Kerkhove was referring to, these are coronavirus carriers who are infected but they never end up showing any symptoms. They feel fine the whole time. And then there's another group called pre-symptomatic. And these are people who've been infected. They haven't gotten sick yet, but they will. And they can definitely spread the virus, we think, up to three days before they start showing symptoms. There's plenty of evidence for this.

SOFIA: OK. So asymptomatic never get symptoms. Pre-symptomatic will develop symptoms.

HUANG: Right, yeah. But here's the catch. You can't tell if someone is asymptomatic or pre-symptomatic until one of them starts showing symptoms. And even if there are asymptomatic people out there that seem to be transmitting the virus less, there are documented cases where it's happened. So what Van Kerkhove says she meant to say is that she has not seen evidence that people who are truly asymptomatic are out there effectively spreading the virus to a lot of other people.


VAN KERKHOVE: And so what we need to better understand is how many of the people in the population don't have symptoms and, separately, how many of those individuals go on to transmit to others.

SOFIA: OK. So let's explain what we know about how many people might be truly asymptomatic - like, just how many of those people there are.

HUANG: The truth is that nobody really knows at this point. Van Kerkhove says estimates she's seen show that anywhere from 6% to 41% of infected people could be asymptomatic. And...

SOFIA: Wow. That's a range.

HUANG: Yeah, it's a huge range. And some studies show that it's possibly even higher. There was this paper that came out last month in a journal called BMJ Thorax, which is looking at the situation on a cruise to Antarctica where more than half of the people onboard got coronavirus. And of those people, 81% of them had no symptoms the entire three weeks they were on the boat.

SOFIA: Whoa.

HUANG: So that suggests that the asymptomatic population could be even higher.

SOFIA: Right.

HUANG: But honestly, this is a really hard group to find and study out there because people usually don't get tested unless they think they might have it. So what we think we know so far, given the limited data, is that it looks like asymptomatic people skew young and also that they're less likely to have health conditions like diabetes or serious asthma that makes them more vulnerable to the virus.

SOFIA: Right. And so for that reason, we don't really know, like, to what extent asymptomatic people are fueling the spread, how much that they're transmitting the virus to others.

HUANG: Right, Maddie. So the research community is actually really divided on this right now. What we have is a few documented cases of some asymptomatic individuals infecting other people. Now, do these represent anomalies, or are they the tip of the iceberg signaling something that happens far more commonly? It's something people are actively trying to figure out.

Van Kerkhove, in her walk-back statement, said that some models are estimating that asymptomatic people could be causing 40% of transmission.

SOFIA: Wow. I mean, Pien, that would be a lot.

HUANG: But still, it could be a lot less. It could be a lot more. It's a huge unknown. And it's also probably really dependent on how people behave.

SOFIA: Do we know how transmission works for people who don't have symptoms? You know, like, if a person isn't sneezing and coughing, how is the virus getting from one person to another?

HUANG: Yeah, right. Like, if you're not actively sick, if you're not running your nose everywhere - spreading coronavirus seems to require situations where people are hanging out really close together, mostly indoors, and doing things that project their voice and breath and spread respiratory droplets. So it's situations like singing in a choir or panting during a dance class at the gym or shouting to be heard in a nightclub. These are all activities that have reportedly led to virus transmission.

SOFIA: OK. So to wrap it up here, Pien, people without symptoms do spread the virus. It probably happens more with people who eventually get sick and have symptoms. And we don't know how much spread is coming from people that never get symptoms because those people are hard to study.

HUANG: Yeah. And this is important. We know for sure that people who don't currently have symptoms of coronavirus can spread it. And it's actually a major difference between the coronavirus that we have now and the coronavirus that caused the SARS epidemic which came through Asia in 2003. With SARS, people really didn't transmit the virus until they were visibly sick or even a few days after.

SOFIA: Right.

HUANG: So we were able to contain that coronavirus by finding sick people and keeping them isolated from everyone else. With this coronavirus, by the time someone knows they're sick with it, it's possible that they've already given it to other people. But the good news - if there is some - is that we have some pretty good tools at our disposal for dealing with it.

SOFIA: Yes, absolutely. Tell them to me, Pien.

HUANG: Yeah. So this might not be brand-new to anyone who's been listening, but it's hand-washing. It's mask-wearing. It's keeping a distance from other people and also definitely, definitely staying home when you're feeling unwell. Those are tools that actually work whether we're talking about asymptomatic or symptomatic transmission.

SOFIA: You know, Pien, I love that advice. It's straightforward. You don't have to understand the tiny nuances between pre-symptomatic and asymptomatic in order to know what to do. And I mean, I don't know. I think that's what's been so frustrating with this whole scenario. It's that the WHO put out some very confusing messaging.

HUANG: Totally, Maddie. It was confusing. And some of that was, you know, unclear messaging from WHO, and I would also say that some of it was the media, too. People picked up a message without adding context and nuances that would help the public understand what they meant. And the language we use around coronavirus is really important. I mean, even right now, everyone's talking about the second wave of coronavirus in the fall, which makes it seem like the first wave of coronavirus infections is over, and it is certainly not.

SOFIA: Yes. I mean, outside of a few states, cases never really went down. Like, we are still in this steady state of infection, and I'm worried that people aren't really getting that message.

HUANG: Absolutely, yeah. And it's even more reason to stay constantly vigilant. You know, you've got socially distance. You've got to wash your hands. You've got to wear a mask - especially indoors because we know that people who seem totally fine can still get each other sick.


SOFIA: All right, Pien, I appreciate you. Thanks for coming on the show.

HUANG: Thank you, Maddie. It's great to be here.

SOFIA: This episode was produced by Brent Baughman, edited by Viet Le and fact-checked by Rebecca Ramirez. I'm Maddie Sofia. Thanks for listening to SHORT WAVE from NPR.


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