Scientists Say Airborne Coronavirus Fuels Superspreader Events

White House outbreak illustrates the risk of an underappreciated Covid-19 infection route, scientists say

Credit: Scott E Barbour / Getty Images

The coronavirus spreads significant distances indoors via invisible aerosols that can linger for hours, building up and causing superspreader events that can infect several people, a group of virus-transmission experts emphasized in an open letter published today in the online version of the journal Science. This airborne transmission is likely the most prevalent path for Covid-19 infections, they say, adding that health officials must communicate this risk more clearly.

“The reality is that airborne transmission is the main way that transmission happens at close range with prolonged contact,” says one of the letter’s co-authors, Donald Milton, MD, an environmental health professor at the University of Maryland and an expert on airborne infections. “You can’t take your mask off at six feet.”

The indoor superspreader risk has been known for months and communicated frequently by scientists since the spring. But outdated research based on science from the 1930s had caused many public health officials and agencies to focus on the six-foot circle of supposed protection.

Even that guideline, considered by scientists to be a helpful minimum at best, has been routinely flouted by President Donald Trump and White House officials, apparently leading to the ongoing outbreak of Covid-19 among people who attended the Supreme Court nomination ceremony September 26.

“One is far more likely to inhale aerosols than be sprayed by a droplet, and so the balance of attention must be shifted to protecting against airborne transmission.”

The six-foot guideline is important, scientists say, but it’s just one layer of protection, a minimum measure that reduces some of the risk of infection, including from larger respiratory droplets that fall to the ground quickly. Aerosols are smaller droplets emitted just by breathing, and more so when talking, and in yet greater quantities if shouting or singing, as well as in a cough or sneeze.

“Viruses in aerosols can remain suspended in air for many seconds to hours, like smoke, and be inhaled,” according to the letter to public health officials and scientists. “They are highly concentrated near an infected person, so they can infect people most easily in close proximity. But aerosols containing infectious virus can also travel more than [six feet] and accumulate in poorly ventilated indoor air, leading to superspreading events.”

“We’ve spent a lot of time talking about social distancing as if six feet is a magical number,” says one of the letter’s co-authors, virologist Robert Schooley, MD, of UC San Diego’s School of Medicine. “The advice we should be giving people is to wear a mask whenever you are indoors with someone other than your family.”

Superspreader events common

Superspreading events, in which one infected person infects dozens of others, have been documented at weddings, in sleepover camps and in churches, choir practices, cruise ships, and other indoor events. Studies have indicated that about 20% of infected people cause 80% of Covid-19 cases, often because of superspreading events. The person at the root of outbreaks may not realize they have the disease because symptoms have yet to develop.

The White House outbreak “certainly has the look” of a superspreader event, Schooley told a group reporters today. It may have originated in indoor gatherings that were part of the nomination ceremony, scientists say, or the infections could have occurred outside, where some of those who became infected were seen hugging others, and few attendees wore masks.

But the risk, scientists stress, is higher indoors than outside.

Aerosols are roughly 100 microns and smaller — there’s no precise delineation between larger droplets and aerosols that can float, and that has been the source of some confusion among health officials trying to translate science into advice. All these droplets, regardless of size, contain saliva and cells from the body, and super-tiny coronavirus particles if a person is infected. They eventually evaporate or land on surfaces, and over time, the virus particles become noninfectious. But studies find this can take hours.

“Individuals with Covid-19, many of whom have no symptoms, release thousands of virus-laden aerosols and far fewer droplets when breathing and talking,” the letter states. “Thus, one is far more likely to inhale aerosols than be sprayed by a droplet, and so the balance of attention must be shifted to protecting against airborne transmission.”

In the letter, the scientists recommend health officials focus more intensely on four preventive measures, which can reduce Covid-19 transmission risk in any setting from the White House to restaurants, schools, or even at home:

  • Moving events outdoors when possible
  • Improving indoor ventilation that helps reduce the concentration of aerosols by bringing outdoor air into a room
  • Upgrading filtration in air conditioning systems with filters that capture aerosols
  • Improving ventilation for high-risk workers

CDC agrees, sort of

Coincidentally, the U.S. Centers for Disease Control and Prevention updated its online documentation today to say “Covid-19 can sometimes be spread by airborne transmission,” an admission omitted from previous documents. But the wording remains vague and at times confusing, says Linsey Marr, PhD, a scientist at Virginia Tech and an expert on the transmission of the coronavirus through the air.

The CDC update distinguishes between transmission by close contact and airborne transmission. But airborne transmission can happen when an infected person is near or far, Marr, a co-author of the open letter in Science, told reporters. “The term ‘close contact’ is somewhat misleading because it suggests that transmission is happening by contact or by large droplets landing on you,” whereas short-range airborne inhalation is probably the dominant means of transmission, she says.

Other scientists agree.

“Aerosol particles are likely the most important transmission route for close contact as they are thousands to hundreds of thousands times more numerous than large droplets and can also penetrate further into the respiratory system,” Richard Corsi, PhD, dean of the College of Engineering and Computer Science at Portland State University and an expert on indoor air quality, tells Elemental. Corsi was not involved in the open letter published by the other researchers.

Layers of protection

“The goal of this letter is to make it clear that the SARS-Cov-2 virus travels in the air and people can become infected via inhalation,” writes Kimberly Prather, PhD, director of the Center for Aerosol Impacts on Chemistry of the Environment in the Scripps Institution of Oceanography at the University of California, San Diego. “It is important to acknowledge this pathway so efforts can focus on cleaning the air and providing guidance on how to avoid risky indoor settings.”

Prather led the group of scientists crafting the open letter. They acknowledge the continued importance of avoiding handshakes and hugs and washing hands frequently — all part of a layered approach to protection that always involves masks and avoiding crowds. Their new message aims to make an underappreciated risk clearer:

“It is important for people to wear masks at all times in public buildings and confined spaces, not only when we can’t maintain social distance,” Marr says. “This isn’t just an academic question, but a point that will help reduce transmission if public health officials offer clear and forceful guidance about this.”

Explainer of things, independent health and science journalist, author, former editor-in-chief of LiveScience and Space dot com.

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