COVID-19 'Superspreaders' Can Exhale Large Volumes of Viral Particles, New Model Shows

By Linda Carroll

July 29, 2020

(Reuters Health) - While many people with active SARS-CoV-2 infections are unlikely to transmit the virus via aerosols, a new modeling study suggests, so-called superspreaders can put out thousands to millions of virus particles per cubic meter of air.

Using previously reported data on viral load and exhaled microdroplet distributions for people coughing and breathing normally, researchers employed a mathematical model to determine the number of virus particles that might be released by low, typical, and high emitters.

Few SARS-CoV-2 infected individuals are likely to have high enough viral loads to transmit the virus via aerosols, said Michael Riediker, founding director of the Swiss Centre for Occupational and Environmental Health, in Winterthur, who coauthored the new study published in JAMA Network Open.

"But the occasional high-viral-load individuals seem to be relevant for this route," Riediker said in an email. "This would explain the low rate of super-spreading events in indoor situations. Already when such a person breathes, concentrations in a room can quickly go up quite high."

Emissions can be boosted if a high-viral-load individual also engages in high microdroplet emission activities, such as singing, shouting, or even worse, combining those activities with physical exercise, Riediker said. "Finally, the emissions from a coughing patient can result even in well-ventilated rooms in airborne concentrations that bring along a serious risk of infection," he added.

Riediker and his coauthor started out with published data on the number of viral copies present in sputum and swab samples from individuals with COVID-19. They combined this information with data from studies on size distributions and numbers of microdroplets exhaled and coughed by healthy individuals.

Plugging that data into a mathematical model, they calculated how much virus an individual would put into the air of a closed, 50 cubic meter room - about the size of a small office - with every breath. The researchers found low and typical emitters didn't put much virus into the air. But high emitters could put out as many as 1,247.7 virus copies/cubic meter when they were simply breathing normally and as many as 7,442,598 virus copies/cubic meter when coughing every 30 seconds.

In a typical hospital setting, with ventilation that exchanges the air in a room 10 times per hour, the virus concentration plateaus after approximately 30 minutes, the researchers note, while in a typical office with three air exchanges per hour, concentrations continue to increase for more than 1 hour, according to the model.

"It seems advisable to avoid situations where you spend extended time with another person in a room, especially if that room is small and poorly ventilated," Riediker said. "Wearing good quality face coverings such as surgical masks can reduce the emissions to less risky levels if worn correctly, at least as long as the person is not coughing."

"If you are in high-risk situations, for example you're a medical doctor attending patients with respiratory symptoms or an at-risk person needing to spend time in a small room with others, you probably should be wearing a good quality respirator," Riediker cautioned.

The new findings emphasize the importance of staying at home if you have symptoms that have been associated with COVID-19, said Dr. Panagis Galiatsatos, an assistant professor in the division of pulmonary and critical care medicine at the Johns Hopkins School of Medicine, in Baltimore. "People without those symptoms can go about a somewhat normal life with a facemask," Dr. Galiatsatos said.

"It also tells me a cough can be dangerous in addition to other things that can cause a turbulent trajectory, such as singing and yelling loudly," Dr. Galiatsatos said.

The take home message from this study should be the importance of continuing to wear a mask, said Dr. Waleed Javid, an associate professor of medicine at the Icahn School of Medicine at Mount Sinai in New York City and director of infection prevention and control at the Mount Sinai Downtown Network.

"It's a very interesting study," Dr. Javid said. "What you don't know if I am standing in front of you and speaking and I have it is whether I am a high or low emitter. And that's why we should continue to wear our masks. And if I am infected I should wear a mask too, so there will be double coverage."

This issue will become all the more important as businesses reopen in New York City, Dr. Javid said. "It's important for us not to let our guards down," he added. "We are in the pandemic of the century. None of us has seen this before and no one can say they know everything about it."

SOURCE: JAMA Network Open, online July 27, 2020.