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Dr. Sanjay Gupta: You’ve probably been hearing the term “super spreader” a lot these days.
In March, a single member of a choir in Washington state infected 52 other people during a two and a half hour practice session. In South Korea, more than a hundred new infections were linked to a 29-year-old man who tested positive after visiting several nightclubs and bars in a single night.
So who, or maybe what, is a super spreader? Why is it so dangerous? And perhaps most importantly — who among us are potential super spreaders?
Elizabeth McGraw, director of the Center for Infectious Disease Dynamics and professor of entomology at Penn State University: Yeah, so the word “super spreader,” it’s a bit messy. I mean, we sort of, for most diseases, have a sense for how many people a single individual is likely to infect. So if someone walked into a room with a bunch of people who were equally susceptible, we ask the question, how many are they likely to infect?
Gupta: That’s Professor Elizabeth McGraw, the director of the Center for Infectious Disease Dynamics at Penn State. I called her recently because I wanted to learn more about super spreader events.
Now before you understand them, you have to first understand how scientists even measure how infectious a disease really is. They do that by starting with a number called the “R-naught” or “R0.”
McGraw: R0 is the number that we use to measure how many individuals you’re likely to infect. So for coronavirus, we think that sits around two point five. And so when we say that average is two point five, what we really mean is that most people are probably only infecting, you know, less than two point five.
But then there are a number of people in the population who are dragging that average up by infecting large numbers of people. There’s not a hard-and-fast rule for when someone becomes a super spreader. So some of the recent studies have used the number of, you know, infecting six to eight individuals as sort of the range, anything above six or eight as meeting the requirements for a super spreading event.
Gupta: Should we be saying that the individual is a super spreader or is it the event is a super spreading event?
McGraw: I think it’s more the event. And, you know, if we want to be really effective and try and stop super spreading, and we know if we could stop super spreading, we’d have a really big impact on this pandemic. We can’t really determine who might be a super spreader or, you know, we really don’t have the knowledge or the resources or the ability to study or understand that. But we do know the environments where they can happen.
And so anytime someone who is positive with the virus puts themselves in a situation where they could infect a large number of people, then you can have a super spreading event regardless of what their underlying biology might be.
Gupta: So anybody could potentially be a super spreader, depending on the circumstances?
McGraw: Exactly. And so I think that’s important for everyone to remember that all of us could be super spreaders.
Gupta: If I had the virus and I was within 6 feet of somebody, is there a way to contextualize, how likely is it that I would then spread that virus to the other person?
McGraw: Well, I think that’s a complicated answer. I don’t think I can give you a number. I think I can tell you that distance matters. You should remember that 6 feet thing is not perfect. So if you’re not wearing a mask and you’re coughing, and someone’s 7 feet away from you, they might also catch it. And also exposure time matters.
Gupta: What makes coronavirus spread so well? What is it about this virus?
McGraw: One thing is that we think that the human population is entirely naive, meaning that they have no immune protection. So that just means the susceptibility is high in humans. Whereas with flu, you know, all of us have some level of protection, be it from past history of infection or from getting the flu shot, which everyone should get.
And then, and I think this is the most powerful thing, is that it can spread in this asymptomatic or pre-symptomatic stage. And in fact, as you pointed out, that is when humans are most contagious. And so it’s very effective at transmitting and getting onto that next host before we’re even aware, which makes it very hard to change your behavior after the fact. In some ways, it’s too late.
Sometimes when people have, by the time they found out they’re positive, and if they haven’t been isolating during that period prior to finding out they’re positive, they’ve probably already infected several people.
Gupta: Yeah, there’s no chance that this just sort of dies away, this virus, right? Because we saw that with SARS, for example, which had a very high fatality rate. I think it was close to 10%. But then it sort of went away. And MERS, the Middle East Respiratory Syndrome. Again, I use these as examples because they’re both coronaviruses, also sort of fizzled away. But that doesn’t seem likely with this.
McGraw: I think, you know, because those diseases were so severe and they were symptoms early on and people went to hospital, and we were able to isolate people, it’s very easy to shut down those transmission chains.
And coronavirus isn’t like that. You know, it transmits before you know it’s transmitted. And so you’re always 10 steps behind it. And so it’s not going to disappear for those reasons.
I know we all wake up in the morning and we have, we’ve slept and we’ve sort of forgotten about coronavirus. We wake up a bit grateful and we think, oh, I wish it would just go away. And then you realize, you know, I don’t think it is going away as much as we’d all like that to happen.
Gupta: Yeah, I’m right there with you. I wake up every morning thinking that. But instead I wake up with my 20 by 20 blue basement room, where I’ve been spending the last several months. There was this recent study from Hong Kong that found that super spreading contributes to a significant proportion of the Covid-19 cases. I’m wondering if you could talk us through that study, I think what they found was this evidence of essentially an 80-20 rule, 20% of people were responsible for roughly 80% of the infections.
McGraw: Yeah. This idea is actually an old one. That any one time, you know, 80% of all the cases of disease in a population can be attributed to anywhere from 20% or less of the entire population. This is a fundamental aspect of many, many diseases. And coronavirus is just the latest example of that.
So the Hong Kong study is, it’s very new. It hasn’t been published yet. And what they did was to look at all of the clusters. So when you’re doing very good contact tracing, you can work out which cases of Covid-19 are related to one another. And so they went and identified around 130 clusters of infections that were related to one another. And then they walked back through them and looked at the path, likely paths of transmission.
And so they found that places like bars and restaurants and weddings were the sort of localities where you had these super spreading events that then led to these very large clusters.
Gupta: So when you put this all together and you look at these super spreading events, you look at this 80-20 rule, data that’s come out of other countries around the world. What does it mean for our public health response here? How does, how does that data help guide our response?
Gupta: I mean, we’ve had more people die in the last 24 hours in the United States of this novel coronavirus, of Covid-19, than other countries have during the entire pandemic. So it is possible doing the things that you’ve mentioned — testing, tracing, isolating, all that sort of stuff to have a huge impact.
Gupta: It’s not perfect, but it can make a huge difference. We’re all in this together. And, and now I think maybe we can add into that we should all behave like we could possibly be a super spreader or be the source of a great amount of spread. So even more incentive to be careful to protect people around you.
McGraw: Good advice, Sanjay.
Gupta: Professor, thank you. I hope we’ll chat again sometime.
McGraw: Yeah. Thanks very much.
Gupta: Now remember, anyone can potentially be a super spreader. There are so many ways that we can be in the wrong place, at the wrong time. But there are also many things we can do to curb this, and the main thing is to really behave like you have the virus. And maybe behave like you could potentially be a super spreader.
So wear a mask, practice physical distancing. It’s the best way to all protect each other. And right now, it’s the only thing we can do to stop these super spreader events.
We’ll be back tomorrow. Thanks for listening.