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What is a coronavirus "super-spreading" event?

Concern grows over "super-spreaders"
Concern grows over "super-spreaders" infecting large groups of people 02:30

A recent investigation showed that dozens of people were infected with the coronavirus during what is known as a "super-spreading" event. After a choir practice in Washington state in March, 52 out of 61 attendees became ill, 32 of whom were confirmed to have COVID-19 and 20 had symptoms consistent with the virus, according to Skagit County Public Health.

The investigation determined that only one person at the practice had "cold-like symptoms" beginning a few days prior to the practice. That individual was later confirmed to have COVID-19, Skagit County Public Health said. 

The choir practice is not the only super-spreading event that has happened during the coronavirus pandemic, and the Centers for Disease Control and Prevention said the potential for these events "underscores the importance" of social distancing and avoiding large gatherings.

Here's a breakdown of what these events are and how they happen. 

What is a super-spreading event?

Super-spreading events "occur when a single person infects a large number of other people — sometimes 10, 20, sometimes even more in one setting," said Dr. Justin Lessler, an associate professor at Johns Hopkins Bloomberg School of Public Health.
Other examples of super-spreading events during the coronavirus pandemic include 15 cases stemming from one person at multiple family gatherings in Chicago and more than 100 cases tracing back to multiple South Korean nightclubs that were visited by a man who later tested positive for the virus.
The term "super-spreader" refers to a person who spreads a disease to a large number of people.

"It's kind of different for different diseases whether it's a person or an event, but for COVID, it would be more like an event," said Dr. Jaline Gerardin, an assistant professor of preventive medicine at Northwestern University Feinberg School of Medicine.
The "world's most famous super-spreader" was Mary Mallon, known as "Typhoid Mary," Lessler said. She infected many people with typhoid fever over several years in the early 1900s. 

How do super-spreading events happen?

These events happen because of "a combination of biology and setting," Lessler said.  

An infected person has to be at a point in their illness when they are very transmissible and they have to be in a place where they can infect a lot of other people. 

Places where people are densely packed and engaging in something that involves a lot of breathing, like singing or talking, are particularly risky for super spreading, Gerardin said.

Could the infected person be asymptomatic?

Yes, the person could be asymptomatic or presymptomatic, the doctors said. 

"They may not feel sick, but the infection has spread enough in their body that they're shedding virus and they're putting enough virus out there to infect other people," Lessler said. 

What is the average infection rate for the coronavirus? 

Doctors have said they believe the number of people that one person with coronavirus typically infects is two to three. That number, however, is considered "over-dispersed," Lessler said.

"When we say over-dispersed, what we mean is there are super-spreading events, so the average might be two, let's say, but most people are infecting one or no people and then a very few people are infecting 8 to 10 or even more people," he said.

The number of typical infections also varies by the density of an area and whether or not people are staying home.

How do we prevent super-spreading events?

The social distancing guidelines many places have adopted help reduce super-spreading events, the doctors said. 

"We can either prevent them entirely by limiting the size of gatherings or we can mitigate a lot of the risk through good prevention," such as having everyone wear masks and being somewhere that's well ventilated, Gerardin said.

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