To Beat Covid-19, You Have to Know How A Virus Moves

It’s technically challenging to actually find pathogens like bacteria or viruses floating around in the air. But Wood has a proxy metric that might at least indicate when a room is potentially riskier to be in. He measures the level of carbon dioxide—figuring that as people breathe in the available oxygen and exhale CO2, everyone else in the room then inhales it, and any airborne pathogens as well.

In technical terms, standards for heating, ventilation, and air-conditioning systems measure ventilation by calculating how often the air in a room swaps out for fresh air from outside—that’s “air changes per hour.” But CO2 levels could potentially be an easier way to alert people if a room needs an air swap for safety. Anything above 1,000 parts per million in an enclosed space, Wood says, would be a sign that it’s time to open the windows or clear out.

Here’s where things get complicated. These variations in small-particle behavior don’t just apply to individuals or single spaces, but how the disease moves across all of them. Epidemiologists are starting to see the spread of Covid-19 not as a cloud or wave moving ineluctably around the world, but instead more like signals moving through a network. The close-quarter, asymptomatic infections that small particles make more likely might also explain the patchy, “checkerboard” spread of Covid-19 through households, cities, and even across the country.

People intuitively think that population density, like in a big city, would lead to more transmission—and at first glance, the massive outbreak in New York City seems to confirm that. But people mostly transmit the virus within their own networks, their own contacts. That’s who the virus jumps to. “I might have basically five to 10 friends that I tend to spend more than 15 minutes in close contact with on a regular basis, wherever I’m living,” Metcalf says. “Yet big cities might lead to more contacts outside of our social networks—casual contacts like commuting may be more frequent, and of course social networks may also be more dense.”

The result, she says, could be “spikier” outbreaks that change with urban forms and even the weather. (Some viruses are seasonal and transmit better in cold, dry air versus heat and humidity, but a sticky August outside can mean cranked-high AC inside, which can actually spread a virus if it’s not filtered correctly.) This is all part of what Benjamin Dalziel, a population biologist at Oregon State University, calls “spatiotemporal heterogeneity,” variability in the way the disease spreads in different places at different times. That spottiness in transmission means that different kinds of public health interventions will be more or less effective depending on when and where they’re used—personal protective equipment and serious ventilation in some settings, rigorously enforced social distancing in others, moving businesses outdoors, continued disinfecting of surfaces, and so on.

That’s how to stop the virus from moving through the world, and it’s what scientists are trying to understand. “It’s about the focal points of transmission within a population, and understanding that some places and times are more important for propagating spread than others,” Dalziel says. Physical distancing measures are blunt tools that address a fundamental prerequisite for transmission, but they are untenable over the long term. “It would be really wonderful if there were one, or a few, factors like that where we could efficiently identify them, make a change, and see a widespread reduction in transmission while still being able to reopen. Wouldn’t that be fantastic?” he asks. “But what’s probably more likely is that there’s a large number of factors that all contribute. There isn’t going to be a magic bullet.”

Meanwhile, the CDC has released recommendations on opening restaurants and schools promoting six feet of space between people, doing things outdoors, and mask-wearing—even as the president says he thinks places of worship should be able to hold indoor services. It can’t be true that large groups gathering in small spaces are dangerous if you’re eating but safe if you’re praying. Even if that’s how the world looks to some people, it’s clearly not the world of the virus.

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