Prognosis: Understanding Silent Spreaders

June 18, 2020

Photo by Nickolay Romensky licensed under CC BY 2.0

A top World Health Organization official sparked controversy earlier this month when she said cases of asymptomatic carriers of COVID-19 spreading the virus are “very rare.” Bloomberg senior editor Jason Gale digs into what we know — and don’t know — about asymptomatic carriers of COVID-19, also known as “silent spreaders.”

This story originally aired on Bloomberg News’ daily coronavirus podcast, Prognosis.

Click here for more of our coronavirus coverage.

Dan Gorenstein: Hi, it’s Dan. We’re really excited to bring you today’s show. While we’re working on reporting out some new stories for later this summer, we still want to make sure you get to hear interesting stories about our health care system. 

Today’s episode was produced by Prognosis, a podcast from Bloomberg News. Prognosis has been providing great coverage of the effects of coronavirus around the world. If you haven’t heard of the podcast, you should really consider subscribing.

In this episode, Bloomberg senior editor Jason Gale digs into what we know —and don’t know — about asymptomatic carriers of COVID-19, also known as silent spreaders.

Jason Gale: Back in February, the Diamond Princess cruise ship was the center of an explosive outbreak of the coronavirus when it was quarantined in Japan. Of some 3,711 passengers and crew on board the luxury ship, almost a fifth caught it. At the time of testing, surprisingly, most of those infected didn’t have fever, dry cough or difficulty breathing, the usual symptoms of COVID-19. And when doctors followed those asymptomatic cases for weeks, they found that just under half never developed any disease. That contradicted initial findings from the World Health Organization. The agency said in March that asymptomatic infections are rare.

But since then, reports from around the world have shown that asymptomatic cases aren’t so rare, and in some instances they might be the source of infections.

Maria Van Kerkhove: The majority of transmission that we know about is that people who have symptoms transmit the virus to other people through infectious droplets.

JG: That’s Maria Van Kerkhove, the WHO’s technical lead on COVID-19, speaking during a Facebook Live event. She was clarifying comments she made about the risk of the virus spreading for people who don’t have symptoms.

MVK: We do know that some people who are asymptomatic or some people who don’t have symptoms can transmit the virus on. 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?

JG: The extent to which these “silent carriers” may also be “silent spreaders” isn’t clear. And it probably varies between groups of people because of factors like age, genetics, underlying medical conditions and people’s individual immune responses. Uncertainty over how the virus behaves has hindered nations’ efforts to reopen battered economies. If there are a lot of people in the community sharing crowded subway trains who don’t know they’re infected, well, that’s a big problem if they’re also contagious.

Dr. Peter Collignon, a professor of medicine at the Australian National University in Canberra, says it’s less likely individuals without respiratory symptoms will spread the virus simply because they’re not coughing and spluttering on other people.

Peter Collignon: I actually do think asymptomatics cause less infections and symptomatics, mainly because they’re not sneezing or coughing as much and therefore they’re not spreading droplets as much. There’s a few studies that show that 80% of cases are due to 20% people, so super spreaders. But that still leaves 20% who aren’t accounted for.

JG: Since infected people without symptoms aren’t in bed nursing their illness, they are typically more mobile, which does present an infection risk. A simple nose rub could transfer virus particles to a hand that could then touch a doorknob or a faucet, which someone else might touch and then rub their eye or mouth, inoculating themselves and causing an infection.

PC: So we’ve got to actually assume that people with minimum symptoms are still a risk for having the virus and being able to spread it. And that actually means that’s good news in the community because it means we can keep on doing a whole lot of things within limits. But we’ve got to just assume it’s there at low levels and therefore take all the precautions we have to not let it spread. So the hand hygiene, the physical distancing, don’t go to work if you’ve got any symptoms etc. That decreases the risk, but it doesn’t make the risk zero.

JG: Peter says that in countries where the coronaviruses spreading out of control, it makes sense for public health officials to target symptomatic cases, isolate them and trace who they’ve been in contact with to monitor for further spread. But in countries where there is little or no obvious transmission going on, it’s important to hunt down all infections.

PC: I think the asymptomatic are still important, particularly if you want to get levels of virus at very low levels of transmission. I mean, if you’re in a country where it’s out of control, concentrate on the symptomatics. I don’t have a problem with that. But if you’re in a situation like Korea, New Zealand, Australia, the asymptomatics are also important to find. And once Europe gets down to low levels, it’s going to be equally important because otherwise you find next winter you’re back in the same problem again.

JG: Infected people can also spread the virus via droplets that come out of their mouths in ways that don’t require coughing. Dr. Sanjaya Senanayake, an infectious diseases physician in Canberra, says that could be important as communities come out of lockdown, especially in the context of large gatherings like protest marches.

Dr. Sanjaya Senanayake: There may be certain scenarios where an asymptomatic person may be creating more droplets or even aerosolizing a bit, for example, in a choir practice. We’ve had outbreaks in a choir setting. In a loud nightclub where you have to shout a lot or a protest or some other event where someone won’t be speaking normally. They might be therefore generating more droplets. And in that situation, an asymptomatic person might be able to spread to someone else that way.

JG: Sanjaya agrees that the likelihood of transmission is lower in a person without respiratory symptoms compared with someone who’s coughing. Problem is, there are a lot of people who are infected without symptoms and their sheer number alone amplifies the risk of some transmission occurring.

SS: There are a lot of asymptomatic infections out there, and we’ll only work out that proportion, ultimately, when we have a good antibody test, so good sero-surveys will give us that answer. And of course, it is kind of hard because is someone truly asymptomatic, if they’re feeling a little bit under the weather or are they actually symptomatic? So there are those those slight subtleties in definition we have to think about.

JG: Sanjaya says the prospect that asymptomatically infected people are less likely to spread the virus doesn’t mean we should abolish physical distancing and other measures to control the pandemic.

SS: Not now, not until we get a vaccine. I think we have to be very careful about that. Low risk is not no risk, and we’ve seen the consequences of what can happen if there is a super spreader event. It has all the downstream effects, the impact on our health system, our economy, socially, it’s just a disaster. So we have to be very, very careful, but still accept that asymptomatic transmission is low risk. But it is still not no risk.

JG: The coronavirus has a way of exploiting our vulnerabilities. When we allow it to spread undetected, cases can snowball quickly. The only surefire way to counter it is to limit its ability to infect others. As the WHO said on Tuesday, in the absence of a vaccine, testing, quarantining and contact tracing remain our best defense against this insidious disease.

DG: That was Jason Gale, reporting for Prognosis, a podcast from Bloomberg News about COVID-19. 

You can check out all of our coronavirus coverage at

I’m Dan Gorenstein, and this is Tradeoffs.

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Episode Resources

Select Research and Reporting:

WHO Now Says Role of Silent Virus Spreaders Remains Unclear (Jeff Sutherland and Corinne Gretler, Bloomberg, 6/9/2020)

WHO Creates ‘Confusion’ About Asymptomatic Spread. Here’s What We Know (Pien Huang, NPR, 6/9/2020)

Asymptomatic Transmission, the Achilles’ Heel of Current Strategies to Control Covid-19 (Monica Gandhi, Deborah S. Yokoe and Diane V. Havlir; New England Journal of Medicine; 5/28/2020)

Episode Credits


Jason Gale, Senior Editor, Bloomberg News

Dr. Sanjaya Senanayake, Associate Professor, Australia National University

Dr. Peter Collignon, Professor, Australia National University

Tradeoffs theme music composed by Ty Citerman. Prognosis music by Leo Sidran.

This episode was produced and mixed for Tradeoffs by Andrew Parrella.

Additional thanks to:

Topher Forhecz, Jordan Gasse-Poore, Magnus Henriksson, Francesca Levy, Rick Schine, Laura Carlson, the Tradeoffs Advisory Board…

…and our stellar staff!