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Superspreaders: These factors affect how fast COVID-19 can spread

When a virus is new and spreads easily between people, such as the novel coronavirus does, the number of cases can quickly rise.

But while, on average, a person infected with novel coronavirus might infect just a few people, there are documented cases of one person infecting dozens of others.

These people – called superspreaders – are individuals who, for one or multiple reasons, may be more contagious than others.

Perhaps the most infamous of superspreaders was Mary Mallon (aka “Typhoid Mary”). Scientists believe she infected more than 50 others during an early 20th Century outbreak as a carrier of typhoid who showed no symptoms.

Similarly, one person who was hospitalized during the 2003 SARS epidemic in Beijing is believed to have set off a chain of transmission that quickly infected 76 patients, visitors and health care workers in the hospital.

So how can one person be so much more contagious than another?

How disease spreads

First, a pathogen – usually a virus or bacteria – needs to be contagious, meaning it can spread from person to person.

One person contracts the illness, and then goes on to infect two others. Those two people then infect four more, and so on, depending on just how contagious an illness is.

In the case of novel coronavirus (COVID-19), the number of people infected with the virus can double or even triple in a matter of days.

When charted on a graph, the numbers look like a curved line getting steeper as it moves to the right. This phenomenon is called exponential spread.

Effect of different activities

Some viruses, such as measles, are highly contagious. Other viruses, such as the virus that causes COVID-19, spread more easily because they are new. When the population has little to no immunity, people are more susceptible to the disease.

When you start with a bug that’s already easy to spread, certain circumstances can make it even easier to spread from person to person.

“The thoughts behind superspreaders is a combination of the number of droplets expelled as well as the environment where the activity is occurring,” said Lori Grooms, director of Infection Prevention and Control, Quality and Safety for OSF HealthCare.

Coronavirus is mostly spread through droplets that contain the virus. Droplets are expelled each time a person talks, coughs or sneezes.

Some individuals may expel more droplets than another person, and some activities are likely to spread more droplets than others. For example, shouting would expel more droplets than whispering.

Likewise, singing could expel more than a person talking in a normal tone, due to the amount of breath or projection behind it.

“The more force behind the activity, the more droplets and even aerosols a person can expel. Aerosols are smaller than droplets. They can be carried further than droplets and can ‘hang’ in the air for extended periods of time,” Lori said.

Timing matters, too

People infected with novel coronavirus may also have a period in which they may be more infectious than other times, as well.

“This would mean they have a higher viral load, or their body is rapidly making more of the virus at a time during their infection. In this case, a person has more virus in their respiratory tract to expel in their droplets or aerosols,” Lori said.

Keep in mind, a person could be most infectious before they develop symptoms.

The environment

The last piece of the superspreader puzzle is the environment.

“How close together are the individuals? What is the air quality and ventilation? Is it an enclosed indoor space, or an open outdoor setting? These are all factors that may play a part in another person’s risk of exposure,” Lori said.

“For example, if you have a person who has a high viral load, that means they already have a lot of virus circulating in their body and they are continuing to make more virus. If that person is shouting in a location that has very little ventilation, there is a higher risk of exposure to contagious droplets to the people around them.”

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Last Updated: February 23, 2024

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About Author: Laura Nightengale

Laura Nightengale was a writing coordinator for OSF HealthCare. 

She has a bachelor’s degree in journalism from the University of Kansas and worked as a reporter at a daily newspaper for five years before joining OSF HealthCare. 

When she’s not working, Laura loves to travel, read, and spend time with her family, including her sweet and ornery dog.

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Categories: COVID-19