Is COVID-19 transmission risk higher in cold winter weather?

Overall outdoor risk is negligible unless it involves close and prolonged interaction, experts say

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      As COVID-19 transmission continues on in B.C., going outdoors this winter is one of the few safe options for social interactions and exercise.

      But this is the first time Canada is experiencing the COVID-19 pandemic during cold weather, and some epidemiologists have raised concerns about increased risk of transmission in cold weather.

      Sumon Chakrabati, an infectious disease physician at Trillium Health Partners in Mississauga, Ontario, says concern over heightened transmission risk in the cold is largely unfounded.

      “It is definitely true that aerosols, air, and vapour and all that kind of stuff behaves differently in the cold,” he says. “But that doesn’t translate into infection risk.”

      Regardless of the weather, Chakrabati says, outdoor gatherings are ultimately less risky than indoor ones.

      “I would much rather have 50 people skating on an outdoor rink than 10 people together watching TV indoors,” he says.

      Looking at the research and data that has been collected over the past year, Chakrabati says the “vast majority” of confirmed COVID-19 cases were contracted via indoor transmission.

      Of the COVID-19 cases that have been sourced back to outdoor transmission, the majority involve indoor settings in some way, such as a barbecue that involves people coming in and out of a house or a semi-enclosed outdoor space.

      He mentions the time in summer when thousands of people were sporadically gathered in Toronto's Trinity Bellwoods Park.

      “If you look at Trinity Bellwoods, obviously, that drew a lot of ire. But there was no signal that it caused any increase in transmission,” he notes.

      Outdoor transmission due to close interactions

      Muge Cevik, an infectious diseases clinician and scientist at the University of St. Andrews, says that other than mixed indoor-outdoor settings, the few other cases where outdoor transmission may have occurred were associated with close interactions, particularly for an extended duration.

      But she emphasizes that colder weather has no impact on outdoor transmission risk of COVID-19 overall.

      “A viral particle has to fly through air, land on mucosal membrane despite the air flow outdoors, cross multiple layers of mucosal defence, e.g. hair follicles and mucus, then infect cells,” she wrote in an email. “Of course, close and prolonged contact indoor or outdoor is a risk factor.”

      Cevik says the biggest threat colder weather poses to increased spread is the fact that cold weather keeps people indoors and reduces ventilation.

      In a Twitter thread on outdoor transmission, she noted that restricting outdoor activities could increase indoor activities with a much higher risk of transmission.

      Chakrabati notes that the focus on outdoor activities and associated risks could be a reflection of people’s ethical interpretation of these types of actions.

      “People have started moralizing what’s right and what’s wrong with COVID, so if there’s anything that seems like it’s like leisure, people will think, 'Oh, look at what that person’s doing—they’re not thinking about the community',” he says.

      “The thing is,” he adds, “if you look at it, it’s more the invisible things that pose a much higher risk of transmission, the indoor dwellings and occupational exposure.”

      The types of activities that most people will be doing outside this winter, such as tobogganing, skating or going for walks, Chakrabati notes, involve continuous movement and relative distance. As such, those are considered very low-risk activities.

      “I would say that overall outdoor risk is negligible unless it involves close and prolonged interaction, or you are in a crowded or semi-outdoor environment,” Cevik says.

      Cevik also notes that focusing on outdoor risk “diverts attention from policies that could have a real impact on the pandemic”, such structural issues causing high infection rates among people working in low-paid jobs and those living in crowded households.

      Ultimately, Cevik and Chakrabati both say the best thing people can do is spend as little time as possible in poorly-ventilated, indoor, crowded settings.

      Spending time outdoors for socializing and exercise purposes is extremely low risk compared to those types of settings.

      “Outdoors, you have basically the best ventilation you could ever get,” Chakrabati says.