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What A Summer Of COVID-19 Taught Scientists About Indoor vs. Outdoor Transmission

Here we are, almost eight months into this pandemic, and it’s like you can’t even invite more than 150 people to sit next to each other unmasked in your rose garden without it turning into a major superspreader event. The White House outbreak is now linked to at least 40 cases, including some that led to hospitalization. As autumn settles in and the holidays loom, it’s worth pausing to ponder what we’ve learned about COVID-19 transmission over the last few months. We’re now entering a time of year when many of us will want to bring our communities together, and when the cold and the dark can increase feelings of isolation. So how do we calculate our own personal risk, while also avoiding … shall we say, bad examples? Just what are we in for this winter, anyway?


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Making predictions about what will happen in the winter based on what has happened so far is simultaneously simple and difficult, experts told me. It’s simple, in that we already know what we need to know to tell that this virus isn’t just going to magically disappear. “I think winter will be bad, because things are already bad,” said Colin Carlson, a professor at Georgetown University’s Center for Global Health Science and Security. From the beginning of the pandemic through Oct. 17, some 217,000 Americans died of COVID-19 and almost 8 million others tested positive for the virus. There’s no reason to assume that the shifting of the seasons is going to reduce the number of overall cases — so fall and winter are likely to go on being bad.

But understanding how seasons can change the pandemic is genuinely difficult. There are a couple of reasons for that. First, while we know that changes in weather have an impact on this virus, there’s a lot of nuance around those effects.

For example, the virus that causes COVID-19 is sensitive to high temperatures and exposure to UV light. But that doesn’t mean weather is the primary thing determining how the virus spreads. “We don’t need cold weather for significant transmission,” said Dr. Daniel Bausch, a virologist and the director of the United Kingdom’s Public Health Rapid Support Team. “It seems to be propagating quite nicely in hot, humid, close-to-the-equator places.” And studies have found that the weather’s impact on the virus probably plays a relatively small part in controlling transmission. In one study, exposure to UV radiation only decreased transmission by about 1 percent. Summer didn’t make the virus disappear, and the chilly weather isn’t, by itself, likely to drastically increase the severity of the pandemic, either.

The other reason it’s difficult to understand the seasonality of COVID-19, though, is that we are missing some key data. Let’s return to the White House outbreak, which was centered around an outdoor event. There’s only been very limited contact tracing done. And there’s not a lot of clear documentation about how people at either event behaved. How many people spent time inside, versus outside? In general, Carlson asked, if people are moving back and forth from inside to out, how do you know where they contracted a virus?

Likewise, said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, we don’t know how much virus makes an infectious dose nor how much exposure is required to reach that point — and because of that, it’s been hard to pin down exactly which circumstances led to transmission. In other words, the difference in transmission between the White House event and crowded bars may not be contradictory at all — it just may indicate that, at the former, people spent more time indoors.

And if there is one thing we can definitively state, it’s that this virus is much, much less likely to spread outdoors than in. For example, in a study of 7,324 Chinese case reports, only two — part of the same transmission event — could be linked to outdoor settings. A database of more than 20,000 cases (including the 7,324 Chinese cases) found 461 that were associated with transmission in completely outdoor environments — predominantly crowded events like markets and rallies. Overall, only 6 percent of all the cases in that database were linked to events that were either totally or partially outdoors. The rest were tied to indoor events. That fact is actually why experts are concerned that fall and winter could lead to an increase in transmission — not because it’s colder, but because people are spending more time inside.

Why does that matter? Some of it has to do with the physics of air movement, said Linsey Marr, a professor of civil and environmental engineering at Virginia Tech. If you’re standing right next to someone else, there won’t be enough time for sun and heat to break the virus down before you breathe it in. But there is enough time for the wind to blow it away. “Imagine someone who is infected as a smoker,” Marr explained. “If someone is smoking, and you want to minimize your exposure, would you rather be indoors or outdoors with them?”

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Air circulation isn’t the only thing at play here, though. After reviewing dozens of studies on the transmission of the novel coronavirus in outdoor settings, Mike Weed, a professor of applied policy sciences at the U.K.’s Canterbury Christ Church University, found that outdoors was safer even before social distancing and mask wearing became the norm. He thinks that is because people just naturally behave differently outside. We don’t pack together like we do in a subway car or elevator. Most adults don’t sit right in their friends’ faces when they’re just having a backyard beer. “Risk outdoors is low regardless of social distancing. But it has to be said that in everyday life, when people are milling around outdoors, people tend to naturally social distance,” he told me. “We think personal space is part of why outdoor transmission is low.”

The situations he found where clusters were associated with outdoor transmission tended to be times when people were, as Weed put it, “invited or required to break natural social distance.” These event-specific behaviors might help explain why Weed found clusters associated with farmers’ markets, where people crowd around the stalls, but not with people just hanging out in the park. And event-specific behaviors could also explain other apparent contradictions — like the fact that protests after a Minnneapolis police officer killed George Floyd didn’t seem to have sparked any transmission clusters.

In that case, you’re talking about events that happened mostly outdoors in crowds. The protests also revolved around chanting and singing and yelling — things we know can spread more of the virus further. But it’s not a normal thing for people to stop by the bar on the way home from a protest, or congregate inside before or after the event. Meanwhile, researchers speculated that the protests and riots that followed may have kept a lot of other citizens who weren’t participating off the streets — reducing the overall number of people who could be exposed.

Because human behavior and air circulation probably matter more than the weather itself, the upcoming holiday season won’t look the same in all parts of the country, Marr said. This isn’t about getting colder, it’s about spending more time inside sealed-up buildings. In June, that meant more transmission in places like Texas, as climbing temperatures forced people into air-conditioned homes and businesses. In October, she said, that could flip-flop, with rates of transmission falling in Texas while they climb in places like Minnesota or Maine.

So what does all of this tell you about your hopes for hygge and holiday get-togethers? If nothing else, it should show you that turning data into personal risk choices isn’t cut and dry, Carlson said. Neither “indoors” nor “outdoors” is universally safe. Indoors with a small number of people and a good ventilation system bringing fresh air in at least six times an hour is a different risk than indoors in a large group singing without good ventilation. Outside at a protest, masked up and walking, is a different risk than unmasked and packed side-by-side into a football stadium.

Where you choose to take your risks, and how, tells you something about not just the science, but your own values.


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Maggie Koerth is a senior science writer for FiveThirtyEight.

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