Surfaces vs Airborne: What We Know Now About Covid-19 Transmission
By: Dana G Smith
Tags: Health, Covid 19, Pandemic, Coronavirus, Six Months In

The Most Likely Way You’ll Get Infected With the Coronavirus

You don’t have to sanitize your apples anymore, but you do have to wear a mask

This story is part of “Six Months In,” a special weeklong Elemental series reflecting on where we’ve been, what we’ve learned, and what the future holds for the Covid-19 pandemic.

At the beginning of the pandemic in March, Jeffrey VanWingen, MD, a Michigan family physician, scared the bejeezus out of people and infuriated food scientists. During his 13-minute video, which went viral on YouTube and has been viewed over 26 million times, VanWingen tells people that when they come back from the grocery store, they should leave groceries outside for three days, spray disinfectant onto each product, and soak produce in soapy water. His rationale was that those items might carry the novel coronavirus and could potentially infect people after they come into contact with them.

Six months later, we’ve learned a lot about how SARS-CoV-2 spreads, and it turns out most of VanWingen’s tips are largely unnecessary and some are flat-out dangerous (you should never bleach your food, but hopefully you already knew that). Instead of obsessing over objects and surfaces, scientists now say the biggest infection risk comes from inhaling what someone else is exhaling, whether it’s a tiny aerosol or a larger droplet. And while a virus traveling through the air sounds terrifying, the good news is there is a safe, cheap, and effective way to stop the spread: wearing a mask. Here are the three primary pathways of transmission, and what experts know about them six months in.

Surfaces don’t seem to matter as much as originally thought

The surface or fomite theory — that you’ll get infected by coming into contact with objects that carry the virus, called fomites, like door handles, shopping carts, or packages — was the original leading contender because that’s how scientists and epidemiologists think most respiratory diseases are spread. For example, when a person sick with a cold coughs or sneezes, tiny snot and saliva particles that carry the virus go shooting out of their nose and mouth and land on nearby surfaces. If someone else touches that surface and then touches their mouth, nose, or eyes they could become infected with the virus. This is why we’re supposed to wash our hands before eating or preparing food, and after using public transportation, or touching door handles, especially during cold and flu season.

“I’m not saying that you can’t get it, that it’s impossible to get it from surfaces, but a very specific set of events have to occur for that to happen.”

Supporting this idea, an early study published in the New England Journal of Medicine found that SARS-CoV-2 survived on various surfaces for several days, including 24 hours on cardboard and 72 hours on plastic. Public health organizations recommended hand hygiene as the first line of defense against the virus, and there were runs on Lysol wipes and hand sanitizer at supermarkets and drugstores, the supply chains for which still have not recovered.

The problem, says Emanuel Goldman, PhD, a professor of microbiology and biochemistry at Rutgers University, is that the experiments those recommendations were based on “were the wrong experiments to do” because they were not representative of how people come into contact with the novel coronavirus in the real world.

“They started out with a humongous, totally unrealistic amount of virus at the beginning of the experiment, and then, sure enough, they found virus at the end. But they started out with so much more than you would ever encounter in real life,” he says. “You would have to have 100 people coughing and sneezing on one small area of surface to get the amount of virus that was used in the papers that reported the survival of the virus on surfaces.”

It turns out that despite the catastrophic harm it’s caused, the novel coronavirus is actually quite fragile, and it doesn’t like being out in the open where it can dry up. According to the NEJM paper, the virus’s half-life is a relatively short six hours, meaning that every six hours 50% of the virus shrivels up and becomes inactive or noninfectious. That means if you start with 100 virus particles, after six hours that number halves to 50 particles; six hours later there are 25 virus particles left, and fewer than 10 virus particles remain after 24 hours. However, if there are huge quantities of virus to start, many more will be left behind after each six-hour window, and it will take longer for all of the virus to disintegrate.

Instead of buying another can of Lysol, maybe invest in an air purifier, more comfortable two-ply cloth masks, or even an outdoor fire pit or space heater.

“If you start out with a realistic amount, and a realistic amount would be between 10 and 100 virus particles, because that’s what a droplet of a cough or sneeze is likely to have, then your virus is gone in a day,” Goldman says. “Now, I’m not saying that you can’t get it, that it’s impossible to get it from surfaces, but a very specific set of events have to occur for that to happen.”

Regardless, it’s critical that people keep washing their hands — although that’s something we should all be doing for normal hygiene anyway — but, Goldman says, you don’t have to do anything excessive, like disinfecting your groceries.

Close range droplets are the new leading theory

In May, the CDC updated its guidelines to state that fomites were not a major source of transmission. Instead, the agency said, the primary route of infection was probably virus-laden droplets — those snot and saliva particles that are at the root of the fomite theory. But instead of worrying about them after they land on surfaces, the bigger concern now is coming into contact with the droplets while they’re still in the air.

When you expel air — whether it’s by sneezing, coughing, talking, singing, shouting, or even breathing — tiny bits of saliva, ranging in size from an imperceptible mist to visible spittle, are pushed out. Heavier particles fall to the ground relatively quickly and are categorized as droplets, while the tinier particles stay afloat in the air for longer. When talking and breathing, the typical droplet trajectory is about three to six feet, hence the six-foot distancing recommendation. If the droplets are expelled with more force, like with a sneeze or a cough, they can travel further before hitting the earth.

Being in close contact with someone raises the risk that you’ll be exposed to the small droplets they’re expelling, and many scientists now think that’s how most people become infected with the virus. One reason is that a virus inside a freshly exhaled droplet is more likely to be alive and infectious than a virus that’s been sitting on a doorknob for several hours. The other reason is that, in close range, breathing in the air that someone else just breathed out is going to expose you to a higher quantity of virus particles — called the inoculum — than after the droplets disperse and fall to the ground.

“It’s not that [surface spread] can’t happen, it’s just that the likelihood is less than if someone was actually right in front of you breathing live virus in their droplets onto you,” says Nahid Bhadelia, MD, an infectious diseases physician and associate professor at the Boston University School of Medicine. “That is a much bigger inoculum, [and] it’s much more likely that there’s a lot more live virus in it, so that’s why it’s a higher risk.”

As a result, social distancing has become one of the recommended ways to prevent transmission, the idea being that if you stay more than six feet away from someone, you won’t be hit by the majority of their exhaled droplets. Supporting this theory, most people catch the virus from someone they live with and presumably are in frequent close contact with. In one study from China, for example, an infected person had a 17.2% chance of spreading the virus to a family member who lived with them, but just a 2.6% chance of giving it to someone outside the home.

“I think people have this preconceived notion that if it’s airborne it’s like the measles or like smallpox where it only takes one viral particle to infect you, and this is almost certainly not the case with this coronavirus. Most coronaviruses are probably in the hundreds.”

However, there have been several documented instances of infections that don’t fit with droplet or surface spread because they happened even when people maintained their distance. Perhaps the most famous example is the choir rehearsal outside of Seattle, Washington, a superspreader event where 52 out of 61 people were infected during a two-and-half-hour practice. What’s notable about this case is that the singers maintained distance from each other and used plenty of hand sanitizer, per safety guidance at the time. Also, the infected person was presymptomatic, so they weren’t coughing or sneezing and projecting droplets further. Despite all this, one person was still able to infect 52 others.

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A study conducted in hamsters in a lab (that’s right, it turns out hamsters are the best animals in which to study coronavirus spread) found similar results in a more controlled environment. The researchers showed that the animals could infect each other not only through direct contact when they were housed in the same cage, but also when they were separated in different cages in the same room. Based on these studies and other mounting evidence, many scientists began to believe that the virus is transmitted through droplets and aerosols, those tiny mistlike particles that can travel farther through air currents and remain afloat for longer.

Aerosol transmission has gradually gained acceptance

Despite these observations, some public health experts were initially reluctant to say that the virus is airborne, partly because they didn’t want to alarm the public. There are also debates between epidemiologists, virologists, and aerosol engineers about what the word airborne really means — whether the size of the particles or their behavior (how quickly they fall to the ground, whether they can be carried on a gust of air) matters more, and what questions must be answered before a disease can be defined as such.

Part of the resistance to calling Covid-19 airborne is also rooted in history. For centuries, doctors and scientists didn’t know how diseases spread. One theory was that infections traveled in invisible clouds called miasmas or “bad air.” It wasn’t until the 1860s that Louis Pasteur’s germ theory of disease began to take hold, cemented in the 1890s with the discovery of viruses. As a result, scientists waged a campaign during the early 20th century to discredit the idea of miasmas and airborne spread with the goal of getting the public to take germs — and personal hygiene — seriously.

“That became the paradigm of epidemiology and infectious diseases from 1910 until now,” says Jose Luis Jimenez, PhD, a professor of chemistry at the University of Colorado, Boulder who specializes in aerosols. “For medicine, during all this time, a disease going through the air is extremely difficult. It’s an outlandish proposition.”

As a result of this legacy, public health experts initially believed that SARS-CoV-2 couldn’t be spread through the air because the presumption was that virtually no diseases were. There have been a few exceptions made over the years, but those were for viruses that are so contagious they couldn’t conceivably be spread any other way — namely, measles and chickenpox.

“For diseases like measles and chickenpox, because they are extremely transmissible, the evidence became too obvious,” Jimenez says. “They’re so transmissible through the air that it just became undeniable, and they were accepted as transmitted through aerosols.”

As surprising as it may sound, by comparison, the novel coronavirus is not very contagious. Each person who gets infected with SARS-CoV-2 will, on average, spread it to two or three other people. A person with measles will infect 15 others. Jimenez says the WHO initially cited the coronavirus’s relatively low infectious rate as a reason why it couldn’t be spread through the air. “[They] are confusing an artifact of history with a law of nature,” Jimenez says. “They are thinking it is a law of nature that if a disease goes through the air, it has to be extremely contagious.”

It wasn’t until a public outcry from over 200 scientists that the WHO finally conceded in July that aerosol transmission was possible.

So if the novel coronavirus is airborne, why isn’t it as contagious as measles? One reason could be that measles is a hardier virus (remember that SARS-CoV-2 is relatively fragile) and can survive longer in those tiny aerosols. Another potential difference is the infectious dose — the amount of virus required to start an infection. Scientists still don’t know exactly how much of the novel coronavirus is needed to make someone sick, but it’s likely higher than conventional airborne viruses.

“What’s the infectious dose via the respiratory route is really probably the last piece of this that isn’t completely answered yet,” says Joshua Santarpia, PhD, an associate professor in the department of pathology and microbiology at the University of Nebraska. “I think people have this preconceived notion that if it’s airborne it’s like the measles or like smallpox where it only takes one viral particle to infect you, and this is almost certainly not the case with this coronavirus. Most coronaviruses are probably in the hundreds.”

Another question that needed to be answered before many public health experts could accept that SARS-CoV-2 was airborne was whether it could even survive in those smaller aerosol particles. Some viruses can’t because they dry up too quickly without a larger liquid droplet to support them. However, many scientists feel this issue has been put to rest with two recent papers (which have yet to be peer-reviewed) that provide what some have called the “smoking gun” for aerosol transmission: live, replicating virus collected from the air of Covid-19 patient hospital rooms.

“Confidently, what you can say is that things that we consider aerosols, not droplets, have both [viral] RNA and [live] virus in them that is capable of replication in cell culture,” says Santarpia, who led one of the studies. “I think that between the two of [our studies], you can say that aerosols are infectious… meaning that probably we’re looking at something that’s airborne.”

How to protect yourself from all transmission routes

By now, most scientists and public health experts agree that SARS-CoV-2 can be spread by both droplets and aerosols, particularly in close range, although no one knows which is the dominant route of transmission. “I think it’s a false dichotomy to think of this as airborne versus droplet. Most things exist on a range,” Bhadelia says.

What matters more is whether people know how to properly protect themselves from the virus. Fortunately, the prevention steps for both transmission routes are largely the same: keep your distance and wear a mask. Evidence of the importance of masks, in particular, has been mounting, not only because they trap outgoing particles from escaping, which protects others, but also because they block larger incoming particles from getting into a person’s airways, protecting the mask wearer themselves. And even if some viral particles do get through, the viral dose will still be much smaller, so the person will be less likely to get seriously ill.

A clear example of the benefits of masks is a recent outbreak in South Korea, in which one woman at a Starbucks infected 27 other customers — whom officials assume were not wearing masks because they were eating and drinking — but none of the employees, all of whom were masked the entire time.

Aerosol transmission does increase the importance of one additional protective step, which is proper ventilation and air filtration. Airflow, either introducing new air into a room or filtering the existing air, can disperse and dilute any infectious aerosol particles, reducing a person’s potential exposure. Being outdoors is the ultimate ventilation, and for months public health officials have recommended that people socialize outside rather than in. However, with winter and colder temperatures coming, indoor air filtration and adherence to masks will become even more important.

“The important thing on the public side is air handling, reducing the number of people in enclosed indoor spaces, and wearing a mask,” says Bhadelia. “[Aerosol transmission] explains why indoor settings are so much more important and contribute so much more to new infections than outdoor settings do.”

Armed with this knowledge, think about how you can make fall and winter safer, both physically and mentally. Instead of buying another can of Lysol, maybe invest in an air purifier, more comfortable two-ply cloth masks, or even an outdoor fire pit or space heater. Be prepared to meet friends outside in colder temperatures or insist upon masks, even in your home. We’ve still got a long way to go before we can declare victory over the novel coronavirus, but at least we know more now than we did six months ago. And you don’t have to sanitize your apples anymore.