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

To really understand how the disease Covid-19 spreads, you have to see the world the way a virus moves through it. It’s just a fleck of protein and genes, a little bit of code in a package with no to-do list beyond hijacking the biology of living things to make copies of itself and spread them to other living things. What happens to those other living things in the process—maybe they get sick, maybe they die—isn’t the virus’s problem. Viruses don’t have problems.sanitation workers cleaning stairs

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If that virus is our problem, though, scientists will want to get in the way of that cycle. Absent a vaccine, understanding that mysterious, turbulent spread is going to be the key to the next phase of the pandemic.

On the long list of changes to society and the way cities will look in a Covid-haunted world, shifting public outdoor space like streets and parking lots away from cars to other uses may be one of the most striking. Multiple cities are instituting “slow streets” or “open streets” programs, to give people more space to be outside while staying six feet apart. Some are going to allow restaurants and other businesses to take over sidewalk and street space for outdoor service, to help them make up the margins lost due to restrictions on indoor occupancy. All of that relies on the idea that disease doesn’t spread as easily outside as it does in enclosed spaces, a relatively uncontroversial notion in epidemiology. But the question of why could turn into the most important countermeasure public health experts can deploy—and it depends on the invisible, infinitesimal particles that come out of people’s mouths with every breath and utterance.

Covid-19 has, so far it seems, three modes of transmission. One route is via surfaces, deposited on things like door handles or silverware that then picked up by someone who touches some entry point into the body—eyes, nose, mouth. In the infectious disease world, those objects are all called “fomites.” They’re why people wash their hands and disinfect surfaces. A second route is through large droplets, like those someone might give off in a cough—or up to 40,000 of them at once in a sneeze, traveling 100 meters per second. They’re bubbles of liquid like saliva and mucus chock-full of virus. Large droplets are much of the reason people think cloth masks are a good idea; a simple cloth mask isn’t a perfect barrier against the output of coughers and sneezers, but if everyone wears one, they drive down overall transmission. Those large particles also provide the logic behind six-feet-apart rules. Even as the force of a sneeze launches those particles outward, gravity pulls them down, though people disagree on whether six feet is the right standoff distance.

That leaves the third, more complicated route. A vast number of the particles that come out of a person’s mouth are much smaller, under 5 microns. They dry out quickly in the air and become so light they can float around for hours. Even the slightly warm layer of air constantly wafting upward from every person—our “thermal plume”—can carry these particles up, up, and away. Random air flow makes their spread turbulent, bounced around by currents like sand in a tide pool. And we emit them all the time. “If you look at what CDC and WHO have been saying, they downplay the role of airborne transmission,” says Joseph Allen, director of the Healthy Buildings Program at the Harvard School of Public Health. “I think that’s a mistake.”
This is basically why people think being outside is less risky than being inside, and it might be why the virus is better at infecting people in enclosed spaces. Given that some significant percentage of disease transmission is coming from people who have no apparent symptoms, it’s still unknown how much virus the different sized particles carry, and how much virus it takes to infect someone. But, given what researchers have seen so far, the chances of infection seem higher inside than out because of how these small particles behave. “The overarching assumption is that the probability of transmission is proportional to the number of virus particles floating around in the air. The more that you inhale, the more likely you are to get it,” says William Ristenpart, a professor of chemical engineering at UC Davis who studies disease transmission. “The room you’re in right now has a roof. Turbulent diffusion goes up and can’t go through the roof. It reflects off. Outdoors, it can turbulently diffuse away.”