What makes a gathering a 'superspreader event'? Experts weigh in

Superspreader events can lead to dozens or hundreds of COVID-19 cases. Experts explain what the biggest risk factors are.
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By Kerry Breen

What do crowded college parties, large weddings, family parties and busy bars all have in common?

All of them have the potential to be superspreader events, where the novel coronavirus can be spread to a huge amount of people at once.

TODAY spoke to two experts about what causes a superspreader event, how they can be avoided and what people should know about 'supertransmitters.'

What is a superspreader event?

While the phrase 'superspreader event' has become shorthand to describe places where the virus has spread to a large number of people, Dr. Dean Winslow, a professor of medicine at the Stanford University Medical Center in Stanford, California, said that the name is not a "recognized, scientific term," and therefore there is no exact definition or qualification for an event to be a superspreader.

"(The term) was really coined this year, as the pandemic took off," Winslow, who specializes in infectious diseases, said. "...It applies to famous events that were linked, epidemiologically, to a large number of cases."

While there is no scientific definition of a superspreader event, Winslow said that events that led to COVID-19 outbreaks have had several factors in common.

"The big concern is that having lots of people jammed indoors, particularly when not wearing face coverings, is sort of a recipe for transmission of SARS-CoV-2," he said. "One of the things that I think is really well appreciated now is that things like speaking and singing will generate very small particle aerosols ... In an indoor environment, where you have less air circulation than outdoors, those particles can remain suspended in the air for several hours or so."

Sten Vermund, Ph.D., dean of the Yale School of Public Health in New Haven, Connecticut, said that the particles produced by singing or shouting can be especially troublesome. Winslow mentioned a choir practice in Washington state where one person spread the virus to over 40 people, despite social distancing; Vermund said that bar and nightclub settings could be a danger as well since people tend to speak louder in those environments.

"While you are speaking you are transmitting," Vermund said. "Keep in mind, too, that a bar or a nightclub tends to be loud, so you tend to be shouting at people. People who are singers or shouters are going to transmit more virus from their respiratory passages to the air than someone who’s speaking softly."

Winslow said that the bar scene seemed to be particularly connected to high numbers of cases, especially since consumption of alcohol may lead to poor decision making.

"There’s also evidence which they can’t quite pinpoint just because we haven’t done such a great job at contact tracing, but many states opened up at the end of May over Memorial Day weekend, where bars were in business and people were not practicing social distancing while indoor dining," Winslow said. "That’s really where that second wave started, in late May and early June, and subsequently some counties have put restrictions on things like indoor dining and bars."

"Some of the bar scenes that have been shown to us on the television are really worrisome," Vermund added. "These are people talking to each other, in the face, in close proximity, no masks, and even a modest viral load in a mildly symptomatic or asymptomatic carrier can transmit under those circumstances."

Other common threads in superspreader events are people being in close contact with each other for long periods of time - like sitting next to each other in church or at a wedding — without appropriate social distancing.

"If you’re at a birthday or going-away party or wedding shower, we’re so socialized into the personal intimacy of those events where the whole point is to get together with family and friends and talk and share and laugh," Vermund said. "Distancing and wearing masks? People find that so burdensome in those environments, but if you forgo those precautions you’re setting up for this kind of (superspreader event)."

To reduce the risk of attending a superspreader event, Vermund and Winslow recommend being outside and limiting the amount of people who you haven't been living with much as possible. Try to make sure that distancing is maintained and masks are worn. If an event must be held inside, keep windows open or use fans or air purifiers to increase ventilation.

"The biggest threat, in my opinion, is an indoor environment where people aren’t wearing face masks or coverings," Winslow said.

Can inanimate objects be superspreaders?

According to Winslow, there's "really been very little documentation" that the virus is "efficiently transmitted" by inanimate objects. While surfaces, especially those that are frequently touched (like door knobs or subway poles) should still be cleaned regularly, he said that the real strategy should be for people to wash their hands regularly.

"It is good that people be very conscious of washing their hands and using hand sanitizer after they touch doorknobs and stair rails and things like that in areas that have a lot of traffic," he said, adding that since the virus is "fairly fragile," those surfaces can be cleaned with simple soap and water in most cases.

What is a 'supertransmitter?'

Certain people may spread the virus more heavily than others. The term for this type of person, according to Vermund, is a "supertransmitter," and it's not a concept that's new to the coronavirus pandemic.

"We can see this in many, many infectious diseases. Two people can be infected, one having a high viral load and the other having a low viral load, and everything in between," he explained. "...The fact that there is a bell curve for coronavirus and some people are on a far-right side with especially high viral loads is no surprise at all. It’s worrisome, because there’s no way to predict who those people are."

While the answer to who can be a supertransmitter may lie in genetic or viral factors, or physical characteristics like the shape of a nasal passageway, Vermund said that there's still no real answer on what causes it.

However, even a supertransmitter can prevent spreading the virus by following appropriate precautions.

"You can have a superspreader who doesn’t actually superspread, if you do practice the core principles of physical distancing, mask use, hand face and surface hygiene, excellent indoor air quality, and preferring outdoor activities," Vermund said.