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Weekly Update On The Science Of COVID-19: Super-spreader Events

Cases of the coronavirus disease continue to rise in Arizona and the country. New research shows just a small fraction of sick individuals might be responsible for the majority of new cases. In KNAU’s weekly update on the science of COVID-19, Melissa Sevigny speaks with infectious disease expert Dr. Paul Keim about these so-called “super-spreader events” and what they mean for controlling the pandemic.

What are super-spreader events?

I think we’ve known for quite a while, we’ve had this sense that there are big events that happen that result in a lot more COVID-19 disease. We’ve heard these stories about in Italy the championship soccer match, here in northern Arizona there was a church gathering that resulted in a large cluster of cases. So there’s these events that are really important…. They’re referred to as “super spreaders.” There’s really two sides to that. One side of it is, getting a lot of people together and having close physical contact so the virus spreads person to person. But the other side of it is the person themselves. In other words, you need a person who is shedding a lot of virus to be in that type of a context.

There’s a lot of new research coming out about these types of events; what have we learned so far?

There’s a really cool study that came out of the London School of Hygiene and Tropical Medicine, they concluded on a global basis, 10 percent of the infected people lead to 80 percent of the cases. Likewise, there was another study done in Hong Kong and they found a very similar number, that a small number of people were infecting a large number. They also concluded 70 percent of the people who were infected never infected anybody else…. We don’t know exactly what it is about particular people that causes them to spread the disease more readily than others. But we do know people tend to be shedding the highest amount of virus right as they’re starting to show symptoms, and then that decreases over the next couple of weeks…. It means a couple of things, first off, we know if people who have symptoms are very judicious about isolating themselves, they can have a big effect on the spread of the disease. The other thing is, it means if we can spot these super-spreader events and people, we can control the disease.

Tell me more about that; how can this help us control the disease?

In Japan they haven’t been quite as strict on the isolation, but they’re focused upon what they’re referring to as the three Cs. What they’re saying in Japan, and I think what I believe as well, is it really takes three things to get these super-spreader events. The first C would be “close contact,” so if you’re hugging people or even just shaking hands, or only a few feet away from somebody who’s shedding virus, that’s a bad thing…. But if you end up in crowds—so the second C would be crowds—that would be an example of where you get these big events happening. And finally “close spaces.” If you’re outdoors and the wind’s blowing 20 miles per hour—that should sound familiar right now—there’s a massive dilution factor on any virus that’s shed from person … So close contact, crowds, and closed spaces are the three C’s that Japan is focusing upon, and by eliminating those situations they’ve done an amazing job of controlling the pandemic without some of the economic impacts we’re seeing in the United States.  

Right, it seems to offer some hope for controlling the disease, if you could just focus on these big events and crowds, that might do it.

I think that’s absolutely true and people can go back to something closer to a normal lifestyle, as long as they’re recognizing that these types of events are critical for controlling the disease. … Again it comes back to personal behavior. You can have a big impact on the health of your friends, family, and community by self-isolating if you’re not feeling good. And likewise you can protect yourself from being infected for the most part by avoiding those types of situations.

Paul Keim, great to talk to you as always.

My pleasure, Melissa.

Melissa joined KNAU's team in 2015 to report on science, health, and the environment. Her work has appeared nationally on NPR and been featured on Science Friday. She grew up in Tucson, Arizona, where she fell in love with the ecology and geology of the Sonoran desert.
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