How you can prevent ‘superspreader’ events this fall and winter

September 16, 2020
to prevent superspreader events, wear masks, avoid creeds, stay apart and gather outside. Some women wearing masks sit at an outdoor table.
Wearing masks and meeting people outdoors are two strategies for preventing superspreader events. Photo: Getty Images.

Superspreader events — where a person with COVID-19 inadvertently infects multiple other people at the same time and place — are likely to increase as we head into fall and winter.

But, you can prevent superspreader events if you follow these relatively simple guidelines from Dr. Daniel Pastula, a UCHealth neuro-infectious disease expert and a neurohospitalist at UCHealth University of Colorado Hospital:

  • Stay home when you are sick.
  • Wear masks as much as possible.
  • Avoid crowds and limit the number of people at events.
  • Stay at least six feet away from people.
  • Maximize “clean” air by holding events outdoors or with good ventilation indoors.
  • Wash hands and frequently touched surfaces as often as possible.

“The risks of transmission aren’t the same across all events. There are some events or circumstances that are very risky for widespread COVID transmission. These are what we call superspreader events and they are largely preventable,” said Pastula, who is also an associate professor of neurology, infectious diseases and epidemiology for the University of Colorado School of Medicine and the Colorado School of Public Health.

Over the course of the pandemic, researchers have learned that the virus that causes COVID-19, SARS-CoV-2, likely spreads both through respiratory droplets and possibly via short-range airborne aerosols. These infectious particles can be very dangerous in poorly ventilated, crowded indoor environments and can lead to superspreader events.

Pastula doesn’t want to single out specific types of businesses or events, but people should be very careful if they work or spend concentrated time in crowded indoor environments.

Concerns about airborne aerosols prompted health experts at the Colorado Department of Public Health and Environment to issue new guidance in recent days about how to properly ventilate indoor areas as well as how employers can prevent outbreaks in workplaces.

A new understanding of how the virus spreads

While health experts and government officials are worried that coronavirus cases are going to spike throughout the fall and winter, Pastula said he’s actually quite hopeful because experts now have a much better understanding of how the virus spreads.

“Since we have a much better handle on how and why superspreader events happen, we have some good strategies to prevent them,” Pastula said.

Dr. Dan Pastula
Dr. Dan Pastula.

The newest theories about the spread of COVID-19 are actually quite groundbreaking, Pastula said.

“Typically, with respiratory illnesses, we usually think of droplets as acting like ballistic projectiles. We previously thought that coronaviruses only spread through these droplets,” Pastula said.

Droplets are relatively heavy. So, when a person sneezes or coughs, the “projectile” droplet is pulled down by gravity within six feet or so and shouldn’t infect people who are farther away. That’s why social distancing — or staying at least six feet apart from other people — is so important. Those who are infected also can sneeze or cough onto their hands, then touch a surface and spread the virus through objects and surfaces.

But, increasing evidence shows that COVID-19 also may travel from person to person via aerosols, much smaller particles that can stay suspended in the air for minutes or hours, and in turn, infect additional people who may be farther away than six feet.

“Aerosols can float. Measles is the classic disease that spreads through the air. It’s so infectious that you don’t need to be exposed to many viral particles to get sick,” Pastula said.

“We’ve always thought there’s been a dichotomy. An illness either spreads through droplets or through airborne transmission. But, with this outbreak, we are learning that it may not be black and white. It’s likely that SARS-Co-V-2 may be transmitted through both infectious droplets and short-range aerosols in high enough concentration to cause infections,” Pastula said.

How a choir practice became a superspreader event

Pastula said lessons learned from two particular superspreader events have been critical to boosting knowledge and preventing other events.

“Now that we know more, we have a much better handle on how this virus has spread,” Pastula said

One of the first known superspreader events in the U.S. occurred in Skagit County, Wash. on March 10, where a choir practice at a rented church hall led to mass infections.

“There were 61 people. One of them had a mild respiratory illness that turned out to be COVID-19. This was early in the outbreak, so no one was wearing masks,” Pastula said. “People were seated close together. They were indoors. They were singing and they were together for about two-and-a-half hours.

“One person is thought to have infected 52 other people in those two-and-a-half hours,” Pastula said. “That’s a secondary attack rate of 87%.”

Since few people knew then exactly how COVID-19 spread, the choir members didn’t know to stay apart from one another. They didn’t know that singing could be especially dangerous, as could speaking loudly, coughing, sneezing and laughing forcefully. All of those actions could cause an infected person to expel more virus particles farther than they would otherwise. Not everyone who attended the choir practice came into direct contact with the sick person. Therefore, researchers believe the virus also spread through airborne aerosols that floated and traveled throughout the multi-purpose room. Ultimately, two choir members died after contracting COVID-19.

Learning from this tragedy can help prevent other tragedies as long as people remain vigilant, Pastula said.

“We know that there are some activities that aerosolize particles more than others. For instance, we think that quiet breathing or talking with mask wearing is unlikely to generate as many infectious droplets or aerosols. However, speaking loudly and singing, shouting and coughing and sneezing all can generate more infectious droplets and aerosols, especially when masks aren’t used,” he said.

Avoid spending concentrated time inside poorly ventilated public spaces

This is where Pastula’s simple guidelines for preventing superspreader events come in.

If the choir member who was sick had stayed home, it’s possible no one else would have gotten COVID-19. If the members had been wearing masks, practiced outside, kept their distance from one another or avoided singing in close proximity, the practice also might not have turned into a superspreader event. But again, not as much was known about COVID-19 transmission back in March.

If fewer people had attended the practice or they had been around one another briefly, rather than for an extended period of time, it’s also possible that some might not have gotten sick.

Pastula said people generally need to be exposed to a sick person for at least 15 minutes in order to get infected, though that time may be shorter or longer depending on the intensity of the exposure.

“The longer you spend in proximity to an infected person, the more opportunity you have to inhale infectious particles, whether they be droplets or aerosols,” he said.

Can asymptomatic people trigger superspreader events?

A second superspreader event that has been well studied occurred in January in China and provided evidence early in the pandemic that asymptomatic people still can spread the virus.

A family traveled from Wuhan — where cases of the new coronavirus originated — to a restaurant in Guangzhou. One family member had no symptoms, but later than day, experienced the onset of a fever and cough. Two other families who sat at nearby tables at the restaurant also became infected with COVID-19.

“One person who was pre-symptomatic infected nine other people in an hour. There was no mask wearing since this was very early in the outbreak,” Pastula said.

The air conditioning was running in the restaurant, so the flow of air could have spread droplets and aerosolized particles from one table to the next. But, the restaurant was large and not everyone there got sick.

The way air particles move is complex. Pastula said it’s possible that the air conditioning system caused aerosolized virus particles to pool in an eddy near the tables where the infected person was sitting. That may be why it didn’t affect everyone else in the restaurant. Or, it’s possible that there wasn’t a high enough concentration of aerosols further away to cause infection at other tables.

Researchers are continuing to learn more about airborne aerosols, but a study in the spring by researchers from the National Institute of Allergy and Infectious Diseases lab in Hamilton, Mont. found that aerosolized virus particles could stay aloft for about three hours.

“This was an experiment in a lab, and might not completely translate to human transmission in the real world. We’re continuing to watch what happens,” Pastula said.

But evidence thus far shows that droplets and potentially short-range airborne aerosols can both be dangerous.

How to stay safe and prevent superspreader events

So, what’s the best way to stay safe and not attend or host superspreader events?

Some of the earliest advice still holds true: wash your hands frequently, avoid touching your face, and by all means, stay home if you have any symptoms of COVID-19, which range from a fever and cough to headaches, body aches, digestive symptoms, loss of taste and smell and in less common cases, swelling of extremities.

Social distancing is also vital because you want to stay at least six feet away from people who are potentially infectious (as droplets usually fall to the ground within six feet and the concentration of aerosols also goes down as a function of distance).

Wearing masks is also absolutely essential, Pastula said. (To see research about wearing masks, click here and scroll to the bottom. Then click on Science of universal mask-wearing.)

“We know that if the infected person is wearing a mask, the mask serves as a control of the source. If they are talking, sneezing, coughing or shouting, the mask catches a lot of infectious particles and prevents them from going as far as they otherwise would,” Pastula said.

Masks also help people who are not sick.

“If you are around someone who is infected, and you are wearing a mask, that may cut the viral dose that a person may get. It’s not 100% effective, but there’s increase evidence that mask wearing helps.”

Pastula said a study from a seafood processing plant in Oregon provided interesting evidence about masks and infections. All workers at the plant were required to wear masks. When there was an outbreak at the plant, universal mask wearing seemed to reduce the severity of infections. Among those who tested positive for COVID-19, all of whom had been wearing masks, 95% were asymptomatic. Researchers believed the masks protected the workers from getting severely ill. They have theorized that they might have been infected with lower doses of the virus and their bodies were able to fight the infection.

“Until we have a vaccine, masks might function as a crude way to help our immune systems recognize and respond to the virus before the body gets overwhelmed,” Pastula said.

How to make indoor air as ‘clean’ as possible

Along with wearing masks, being mindful about ventilation is absolutely essential.

“It’s important to ‘clean’ the air as much as possible. How do we do that? First, if you can hold an event outside, that’s best. There’s so much ventilation outside. With wildfires, that may not be possible, but when air quality and weather allow, gathering outside is always better.”

When inside, it’s also possible to keep air as clean as possible. Whenever possible and safe, open a window to keep air moving.

Heating and cooling systems also can provide help. Most furnaces and air conditioning units have filters.

Change your filter as recommended, and use the highest level of filtration which is compatible with your system. Click here to see a guide from the Environmental Protection Agency on how to keep indoor air safe at your home.

Pastula recommends using filters that are rated at MERV 13 or higher. He also suggests running your heating and cooling system in a way that brings outdoor air inside as much as possible.

You can also keep your system running all the time.

“There’s a circulation setting on most HVAC units. You’re not heating or cooing, but running it in the background,” Pastula said. “Running it at a low level all the time, rather than turning it on and off allows the air to continuously run through your HVAC filter.”

Managers of commercial buildings also must be mindful about the safety of indoor air and many are making adjustments as the seasons change.

Until the pandemic eases, avoid crowds

Along with working to keep air cleaner, in order to prevent superspreader events, we are all going to need to keep avoiding crowds.

“The highest risk of a superspreader event would be lots of people from different households gathering indoors for more than 15 minutes, not wearing masks, with a lot of loud talking and poor ventilation,” Pastula said.

“This fall and winter, we’re going to have to keep being adaptable. Be really smart,” Pastula said.

“COVID-19 is still circulating. We are not out of the woods yet.

“Gather with fewer people. Try to be outdoors if you can. If you can’t be outside, make sure the ventilation is good indoors. Wear masks, except when actively eating and drinking. Spread out. And make sure anyone who is sick stays home.”

About the author

Katie Kerwin McCrimmon is a proud Coloradan. She attended Colorado College thanks to a merit scholarship from the Boettcher Foundation and worked as a park ranger in Rocky Mountain National Park during summers in college.

Katie is a dedicated storyteller who loves getting to know UCHealth patients and providers and sharing their inspiring stories.

Katie spent years working as an award-winning journalist at the Rocky Mountain News and at an online health policy news site before joining UCHealth in 2017.

Katie and her husband, Cyrus — a Pulitzer Prize-winning photographer — have three adult children and love spending time in the Colorado mountains and traveling around the world.