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.
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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.”
This has held true for Covid-19. In a not-peer reviewed preprint from April, a team at the University of Hong Kong and in China found that of 318 outbreaks in China, none occurred outdoors. In another April preprint, government and university public health researchers in Japan assessed 110 individual cases in 11 Covid-19 clusters, and found that the odds vastly favored transmission indoors. Researchers also have a growing number of indoor-cluster case studies, like the Diamond Princess cruise ship; a restaurant in Guangzhou, China, where people sitting in the exhaust from an air conditioner got sick but others didn’t; a call center in a Seoul skyscraper, where almost everyone one one side of the office got sick but not on the other; a Washington state choir practice; and a cluster centered on a biotech conference in Boston. The risk of going to an outdoor cafe or walking on widened, car-free streets seems much less than, say, working in a meatpacking plant or going on a cruise ship vacation.
Disease models tend to regard the number of people a given person infects, the reproductive number, as a sort of average across an entire population. But that number actually varies by individual and by context. Most infected people don’t transmit the disease to anyone else; so-called superspreaders give the disease to lots and lots of others. Jessica Metcalf, a demographer who studies infectious disease dynamics at Princeton, estimates that 10 percent of cases might be responsible for 80 percent of transmission. Some people apparently walk around in an invisible Pigpen cloud of virus. And some circumstances—crowded rooms, sick people exuding more virus, longer periods of contact—make some situations more likely to turn into “superspreading events.”
The virus lives in the deep lungs, and has to get up and out of the nose and throat for transmission. Some researchers looking into the Washington choir cluster suggested that the deep breaths and powerful exhalations required for singing carried more virus, making that outbreak worse (even though it turned out that the singers had in fact gotten inside each other’s social-distance force fields). Ristenpart’s team at UC Davis has found that simple talking gives off 1 to 50 particles per second, with louder talking corresponding to higher numbers. That might be due to something called the “fluid-film burst mechanism”; when you inhale, the air-gathering sacs of the lungs, the alveoli, expand and stretch the thick fluid that lines them. It pops and splashes a bit, pinching off the tiny aerosol particles. And that part of the lungs is exactly where the virus is more likely to live too. “The physicists have accepted this,” says Robin Wood, director of the Desmond Tutu HIV Foundation in Cape Town, South Africa and an expert in the airborne transmission of tuberculosis, “whereas the physicians haven’t really got to understand it.”
And some people give off more of these “expiratory” particles than others—by an order of magnitude—no matter how loud they talk or how deeply they breathe. “A 10-minute conversation with an infected, asymptomatic superemitter talking in a normal volume thus would yield an invisible ‘cloud’ of approximately 6,000 aerosol particles,” Ristenpart’s team writes in a paper in Aerosol Science and Technology. They’ve even found that some sounds emit more of these expiratory particles than others—pa-pa-pa (linguistically a “plosive”) makes more than fa-fa-fa (a “fricative”). All in all, it’s a good argument for quiet-car rules on trains and buses in the Covid-19 era.
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But it still takes context to turn that individual variation into a superspreading event. If small expiratory particles are a major factor, then a superspreader’s kryptonite will be ventilation. Work on the airborne indoor transmission of tuberculosis in the 1950s showed that the outcome depended on the number of people who were infectious, their respiratory rate, and how well the room was ventilated. It’s that last number that may provide another angle of attack for public health.
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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