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How safe are outdoor activities?

MIT Medical answers your COVID-19 questions. Got a question about COVID-19? Send it to us at CovidQ@mit.edu, and we’ll do our best to provide an answer.

Now that Delta is the predominant variant in the U.S., is it safe to be unmasked during outdoor activities where I might end up in close proximity to other people — other walkers in my neighborhood or other cyclists or pedestrians when I’m riding my bike on a bike path? And suppose I want to walk or ride my bike with a friend? Or attend an outdoor gathering? While I don’t know the vaccination status of people in my neighborhood or strangers on the bike path, my friends and I are all fully vaccinated, if that makes a difference.

Illustration of a person running down a residential street; a dotted line shows their winding path around several people

We’ve been getting a lot of these “Now that Delta’s here, is it safe to…” questions.

So, let’s start by saying that, during a pandemic, no activity involving interaction with other human beings is “safe.” But risk is relative, and activities that we pegged as low-, moderate-, or high-risk a year ago, when we first began talking about risk assessment, still maintain their relative positions on the overall scale of COVID-19 risk. Even if the more transmissible Delta variant has ratcheted up those risk levels by a few notches.

Outdoor transmission of the virus is rare. You might have heard that less than 10 percent of documented viral transmission happens in outdoor settings. That “10 percent” figure has been tossed around a lot, even by the CDC. It comes from a comprehensive analysis of relevant research published last November, but most experts, including the authors of that paper, believe that the actual risk of outdoor transmission is far lower — likely less than one percent. That higher “conservative estimate,” the authors explain, included several studies in which the exact location of viral transmission was unclear — like five cases of transmission among construction workers that could have happened outdoors but might as easily have occurred in an enclosed portion of a building under construction. Or an outbreak of COVID-19 at a summer camp in Georgia that was clustered by sleeping-cabin assignments but where outdoor transmission could not be completely ruled out.

Less ambiguous studies show just how infrequently the virus is transmitted outdoors. For example, a Chinese study done in the early days of the pandemic found just one case, out of more than 7,300, connected to outdoor transmission. A Japanese investigation of 110 cases found the probability of transmission to be 18.7 times higher indoors compared to an “open-air environment.” And a more recent study, which looked at transmission between 18 infected construction workers and 496 of their close contacts, showed that the infected individuals were nearly 25 times more likely to spread the virus to coworkers in enclosed spaces compared with outdoor settings. They transmitted the virus to 26 percent of their indoor coworkers while infecting only 1.4 percent of their outdoor workmates — this despite being significantly more likely to share meals and talk loudly while working outside.

When outdoor transmission has been shown to occur, it’s almost always been associated with lengthy, close interactions — the riskiest type of interaction in any environment. Even with the more transmissible Delta variant, fleeting outdoor interactions, like passing someone on the street or on a bike path, carries negligible risk.

But the virus isn’t the only thing that’s changed since we wrote about risk assessment last fall. As you point out, many of us, including you and your friends, are now fully vaccinated. Study after study has demonstrated vaccine effectiveness, even against viral variants like Delta, and activities with other fully vaccinated individuals carry very little personal risk. While breakthrough infections are more likely with Delta, you’re well protected from serious illness. If you do become infected, you’re likely to be only mildly ill — and you might not have any symptoms at all.

But risk calculations get trickier when you’re likely to be in close proximity to unvaccinated people for any length of time, even outdoors. While vaccinated people, like you and your friends, are safer now than at any time since the beginning of the pandemic, unvaccinated and immunocompromised people are in more danger than ever before. Children too young to be vaccinated are becoming sick and being hospitalized in higher numbers. And because there’s evidence that vaccinated people who become infected can spread the virus, your risk assessments need to include both your probability of being exposed to the virus and the possibility that, in the unlikely case of a breakthrough infection, you might subsequently expose an unvaccinated child or vulnerable adult.

That means thinking beyond your immediate circle of close contacts. What’s the vaccination rate in your area? How about the positive-test percentage? The risk of almost every activity involving other people — even outdoors — will be greater if you’re in a place where vaccination numbers are low and positivity rates are high or increasing.

While everything might have seemed more straightforward a year ago when we were all being asked to follow the same guidelines and take the same precautions, Medical Director Cecilia Stuopis warns against making things more complicated than they are. “Get vaccinated, wear a mask indoors, choose outdoor over indoor activities, and avoid obviously risky situations. Even if your individual risk is relatively small, make choices with the goal of reducing risk for the vulnerable people in our community,” she advises. “It comes down to common sense, personal responsibility, and controlling the risks you can control while continuing to work, study, and live your life,” she adds, “because the virus isn’t going away.”

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