The realities of coronavirus ‘super-spreading’

It’s a phenomenon seen at Boston’s Biogen meeting and elsewhere. What can we do about it?

A Biogen leadership conference at the Boston Marriott Long Wharf hotel in February became an epicenter of the coronavirus epidemic in Massachusetts.David L. Ryan/Globe Staff

More than a century ago, Mary Mallon, a New York cook, was suspected of transmitting typhoid fever to 51 people, earning her the notorious nickname Typhoid Mary.

At the Biogen leadership meeting in Boston last month, a different kind of infection spread with lightning speed, leading to at least 99 coronavirus cases in Massachusetts and more around the world.

What do these cases have in common? They’re both classic examples of what epidemiologists call “super-spreading.” And the phenomenon has major implications for the way the current coronavirus pandemic is playing out.

For decades, scientists and doctors have known that in epidemics, a small number of people can spread a disproportionately large number of infections. It’s been documented with infectious diseases from SARS to measles to Ebola, and already, multiple cases of super-spreading have been reported with COVID-19.

Biogen’s two-day meeting in Boston has become infamous as the epicenter of the coronavirus spread in Massachusetts. It is not known who among the approximately 175 executives from across the country and around the world brought an infection with them and shared it with so many. But already 99 infections in the state have been linked to attendees at the biotech gathering. That event accounted for more than three-quarters of all documented infections in the state two weeks ago, though as of Thursday it was responsible for just 4 percent of all 2,417 confirmed Massachusetts cases, as more people unrelated to Biogen have tested positive.

Other Biogen-related infections or suspected cases have popped up as far away as Norway and Argentina.

Media reports about a super-spreader party in Westport, Conn., after which at least 22 of about 50 partygoers tested positive for COVID-19, and a super-spreader British businessman who may have become infected in Singapore and unknowingly passed the virus to at least 11 people, have helped popularize the phrase.

But it’s an idea more than a century old, dating back to when Mallon was suspected of transmitting typhoid fever to 51 people in the early 20th century, possibly by failing to wash her hands thoroughly before serving up ice cream desserts.

“The term has kind of — you could say it’s gone viral,” said William Hanage, an associate professor of epidemiology at Harvard’s T. H. Chan School of Public Health.

What might cause such super-spreading in today’s pandemic? Doctors and scientists are still learning about this coronavirus, and it’s too early to draw conclusions about it, they say. But based on previous epidemics, a range of circumstantial, biological, and behavioral factors might contribute to why some individuals, regrettably, transmit an unusually high number of infections.

In some cases, it’s just because someone happens to make more contacts at a particular time. “If the person becomes highly infectious at the point they go to a bar to celebrate St. Patrick’s, say, then it can transmit to an extremely large number of people,” Hanage said.

In most cases, the circumstances in which people find themselves constitute the most powerful predictor of whether they might turn into robust vectors of infection, said Dr. Megan Murray, a professor of global health at Harvard Medical School. “Bars are amazing super-spreading environments,” she said. Or in a high school, one student going from class to class might easily come in contact with 100 people in a day. “Anybody who’s at school and who is infected could be a super-spreader just because of the very fact that they go to school,” she said.

Crowding or weak ventilation could help cause super-spreading events. And of course, some people are more naturally gregarious, with lots of friends they like to stay in physical contact with, offering an infection extra chances to spread.

Other reasons could be simply biological. A person “could have a higher viral load, meaning that there’s more virus in their lungs,” so when they cough it leads to more possible infections, said Murray.

Paradoxically, just as a very sick person might infect a lot of people, so might a not very sick person, she said.

People who are infected but asymptomatic usually have a lower viral load, she explained, but could still be contagious and might spread infections with unguarded behavior. “If you’re perfectly healthy and you don’t have any idea that you’re shedding virus, even if you’re not shedding very much virus, you’re going to the coffee shop and shaking hands with people and kissing your friends,” she said. “The fact that you’re not that sick might be a reason to spread the infection widely.”

It remains unclear how exactly coronavirus leaped from person to person during the Biogen meeting. Researchers say the coronavirus is generally spread via respiratory droplets and possibly aerosolized particles.

On Feb. 26, attendees shared breakfast and lunch at the Boston Marriott Long Wharf hotel, and they congregated in the hotel’s Grand Ballroom for sessions with titles like “Orchestrating Agility in the Matrix,” “Where Science Meets Humanity,” and “Living up to Our Purpose.”

That evening they rode the elevator to the 33rd floor of 60 State St. and strolled into the State Room, a lavish space with sensational views of the city and Boston Harbor, for dinner and an awards celebration. There were hors d’oeuvres and drinks, with bars set up at either end of the room.

“Everyone was in a good mood,” recalled a person who was there, who requested anonymity out of concern for potential career consequences. “Everybody was professional. It was a group of happy colleagues having a nice evening.”

One of the only downbeat notes was that it was foggy that day, so nobody saw the amazing views, said the person, who later tested positive for COVID-19 and — after a self-quarantine — has since recovered.

“It wasn’t some orgy,” the person said. “There was zero unusual about it.”

Such mundane occasions can easily lead to the spread of disease, epidemiologists say — which is why social distancing is so vitally important now.

“There are all these ways in which we make contacts with people that we don't think about as being potentially transmitting an infection,” Hanage said. “And because we usually have immunity to things, we're not so concerned about it as we would be at our best.”

The Harvard epidemiologists noted that they prefer not to use the term “super-spreaders.” It stigmatizes people, according to an official for the World Health Organization, which avoids the term. It’s also not a useful concept from a scientific standpoint, said Murray.

“You don't want to be blaming people who have unknowingly done this,” Hanage said.

Instead, the epidemiologists emphasized, it’s more useful to refer to “super-spreading events” and examine the kinds of circumstances that lead to them. Then we can all do our best to avoid those circumstances by practicing social distancing and staying away from places where super-spreading is more likely.

In other words, it’s not necessarily the Kevin Bacons of the world, with their six-degrees-of-separation connections, who are responsible for super-spreading. Under the right circumstances, any of us could be.

“I think the thing which is most important to understand is that while super-spreading events are important, especially in initiating a large outbreak, you mustn’t forget about the ordinary, everyday things,” Hanage said. “Even if you only infect one other person, who’s to say that that one other person isn’t themselves going to end up contributing to a super-spreading event?”

Andy Rosen of the Globe staff contributed to this report.