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How is the coronavirus transmitted from person to person?

Seven months since we discovered the virus, we have learned much more about how it spreads. But there’s still significant controversy on some aspects of the research as public health experts debate how best to inform the world.

I want to show you it all: the stuff we know pretty well and the stuff scientists fight over. To start, let’s break down the three major modes of transmission.

Surface transmission

Scientists call it: Fomite transmission

Everyone agrees the virus can live on surfaces and objects. When we’ve tested hospitals where they treat COVID-19 patients, we find the virus on the floors as well as the shoes of the doctors and nurses who walk on them. We find it on the computer mice used by staff. We find it on the sickbed handrails and in the restrooms patients use. It’s on the window ledges and doorknobs, patient cellphones and remote controls. You get it: The stuff gets everywhere.

And we have evidence the virus can last for a relatively long time. On tissue paper, it’s only a few hours. Wood? Two days. Glass? Four days.

But what we don’t have are many examples of someone getting sick from touching a surface and certainly not on a huge scale. One of the most famous cases of coronavirus transmission came from Germany, where a man was infected by sitting back to back with someone who had it. Their only contact was when one asked the other to pass the salt shaker. Even that case can theoretically be explained by other modes of transmission: What if small respiratory droplets from the infected man found their way to the susceptible man through even that short interaction?

There’s also been a case that a North Carolina epidemiologist traced back to a pharmacy keypad.

And... that’s about it. There are probably more obvious individual cases of surface transmission, but the truth is that it just doesn’t happen frequently. As one report stated: “There are few to no clear cases of COVID-19 [surface] transmission found in the literature.” In particular, the virus seems to be not particularly infectious in small surface doses. Unless someone sneezes on a counter, you touch it with your hand and then touch your mouth or nose, you’re probably fine.

The Centers for Disease Control and Prevention and the World Health Organization, looking at both public and private data, came to the same conclusion. “It may be possible that a person can get COVID-19 by touching a surface or object that has the virus on it and then touching their own mouth, nose, or possibly their eyes,” the CDC writes. “This is not thought to be the main way the virus spreads.”

This one isn’t controversial: Nearly everyone who has looked at the subject agrees that surface transmission is rare.

Spittle transmission

Scientists call it: Droplet transmission

Spittle transmission is what the CDC and WHO consider to be the major way the coronavirus is spread. It involves droplets being expelled from one person and then landing in or near the mouth, nose or eyes of another.

Of course, the amounts of droplets released and the distance they travel are pretty dependent on how they’re released. Normal breathing? They’ll go less than 3 feet. Normal talking? Slightly more. Coughing? Over 7 feet. Sneezing can be really dangerous: Droplets are expelled at more than 100 mph and can end up traveling 20 feet or more.

Of course, there’s significant variance here: Heavy breathing and loud talking probably shoot the droplets farther. These droplets move ballistically — that is, they’re primarily dependent on their initial speed and the force of gravity pushing them downward.

But the agreement by the CDC and WHO that this is the major form of transmission has not led to identical social distancing guidelines. The CDC’s 6-foot rule is famous locally, but the WHO still only recommends one meter, just over three feet. Australia splits the difference and says 1.5 meters.

Like everything, it’s not black and white but a gradient. You’re going to be exposed to more spittle at 3 feet than 6 feet, and more at 6 feet than 20 feet.

As a side note, one study shows that kids are less adept at exhaling droplets than adults. Not only are the number of droplets released low, but kids are releasing them lower to the ground than adults. Less height means that those droplets go less far and are less likely to end up landing on the face of an adult. That might explain why younger children don’t seem to be as infectious as older children and adults.

Mist transmission

Scientists call it: aerosol or airborne transmission

Think about when you see your breath condense on a cold day. What happens? The cloud doesn’t fall to the ground immediately, but lingers in the air.

Even after those droplets become invisible, they remain afloat for some time. They hang out until wind pushes them away, or they evaporate, or they combine with water droplets already in the air to create bigger droplets and fall to the ground.

These are what infectious disease specialists call aerosols. They are small droplets, and because they’re so small, they don’t fall immediately like raindrops, but hang around like fog. And if people can be infected by them, closed spaces can be dangerous for people who haven’t had close contact with an infected person.

Here’s maybe the single most controversial question in coronavirus scientific circles right now: to what extent is COVID-19 transmission due to these aerosols?

The WHO doesn’t currently consider aerosol transmission to be a threat. It notes “short-range aerosol transmission cannot be ruled out,” but generally the organization “had steadfastly pushed back against the idea that there is a significant threat of the coronavirus being transmitted by aerosols,” Nature magazine wrote. The CDC flat out doesn’t mention aerosol or airborne transmission in its latest public info sheet.

This has made a large group of scientists confused and upset. A letter signed by 239 infectious disease specialists urged the public health organizations to address the issue of aerosol transmission. Honestly, their research is logical and clear cut, pointing out the large numbers of cases where you can’t explain them via big-droplet theory alone.

A restaurant in Guangzhou, China, is a key example: The virus spread around multiple people significantly beyond 6 feet away likely because of airflow. On buses where one passenger infected 24 others, there was no correlation between the distance to patient zero and the probability of infection. To explain the Skagit County choir outbreak, you’d have to believe the initial infected person rained big droplets on a set of chairs 30 feet wide, singing sprinkler style. Ditto with the outbreaks at places like prisons and meat-processing plants, where people who don’t work in close contact clearly gave one another the disease.

Aerosol transmission being a major player also would explain why outdoor transmission is so much rarer than indoor transmission. After all, if we’re talking droplets that ballistically land on someone, those are going to work just as well outside as inside.

I understand why the CDC and WHO are skeptical. The most famously aerosol transmitted disease is measles, where being in the same room as a contagious person is usually enough to get it. That’s clearly not always the case with this coronavirus, though it is sometimes. We’re very lucky the coronavirus isn’t as contagious as measles.

But aerosol transmission doesn’t have to be black or white: Different viruses could live for different lengths of time in these aerosols. If enough are emitted — like while singing, or yelling at a meat-processing plant at low temperatures — aerosol transmission could occur. We’ve seen the CDC and WHO drag their feet on research before with masks, and it proved to be a big mistake.

For someone to believe aerosol transmission isn’t a major concern requires “contortionist thinking.” Now, I think you can haggle over the percentages here. One scientist guessed 98% of transmission was due to aerosols, but that feels too high for me based on the documented spread I’ve seen. Jose-Luis Jimenez, an aerosol specialist at the University of Colorado, opines that a majority of transmission was due to these aerosols. Other scientists think it is a major player but somewhat less than 50%.

To help people understand aerosol transmission, Jimenez created a risk assessment calculator based on what we know about how aerosols fill a room. We know how many aerosol particles people release during different activities — breathing, speaking, exercising — and how long those are likely to last in the air at various temperatures and humidities. Add in the size of the room, how well the air is circulated and filtered, whether people are wearing masks, etc. In the end, the calculator spits out a rough estimate of how many people are likely to be infected in various conditions. Keep in mind: Jimenez’s calculator assumes social distancing throughout.

He tuned it using real-life examples. If you put the Skagit County choir practice particulars in there, the calculator will spit out that 53 people are likely to be infected, just as in real life. However, if that same practice would have happened outside, Jimenez’s calculator estimates that only 0.5 people would have been infected, thanks to the rapid exchange of air that happens outdoors. The calculator’s presets include classrooms, mass rallies, subways, buses and more, but you can put your own home’s details in there and see what happens.

Interventions that make sense

Understanding how the virus is spread is obviously crucial for determining what safety measures we choose to enact.

On one hand, the “hygiene theater” of cleaning every surface known to man with cleaning products probably isn’t super effective. Keeping things clean is generally a good idea, don’t get me wrong, but in terms of how the coronavirus is spread, cleaning surfaces is not the answer. When Utah closes liquor stores for sanitation after an employee tests positive, the major bonus from a coronavirus point of view comes from cycling the air in and out a few times and quarantining employees who had close contact with the infected person, not from cleaning every wine rack.

Masks are important, though, in that they filter nearly all big droplets and a percentage of aerosols. N95 medical masks are best, but even poorly fitted cloth masks might have a bonus of 30% to 50%. Face shields are likely to add good protection for droplets but not very good protection for aerosols.

Regardless, it’s clear that masks really make a difference. One infected Chinese patient took two bus trips on the same day. On the first trip, he didn’t wear a mask, and five people were infected. On the second, he did and zero people were infected. In Japan, nearly everyone wears a mask on the famously crowded train; as a result, “transmission on the train is not common.”

Also important are ventilation and filtration indoors. Remember, how long the virus lasts indoors is dependent on airflow; even opening a small window can be very effective.

How long virus-containing droplets last in various indoor ventilation scenarios. https://www.thelancet.com/journals/lanres/article/PIIS2213-2600(20)30245-9/fulltext

ASHRAE, the American Society of Heating, Refrigerating and Air-Conditioning Engineers, has a frankly wildly impressive list of best practices in nearly every indoor situation, including a 41-page guide on reopening schools.

We all have limited budgets and time to handle the coronavirus. So let’s do the best we can with those resources, and actually apply them to interventions that make sense: masking, ventilation, filtration and staying away from others.

Andy Larsen is a Salt Lake Tribune sports reporter who covers the Utah Jazz. During this crisis, he has been assigned to dig into the numbers surrounding the coronavirus. You can reach Andy at alarsen@sltrib.com or on Twitter at @andyblarsen.