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On March 10, a choir in Washington State chose to proceed with its rehearsal despite increasing concerns about the spread of the coronavirus in the U.S. The session, which was attended by 61 of the usual 122 members, lasted for two and a half hours, with participants taking care to limit interactions. Ultimately, an estimated 87% of those present became sick, according to a recent CDC report on the event.
'The choir practice serves as one of the most notable examples of a super-spreader event,' says Prathit Kulkarni, M.D., an infectious disease specialist at Baylor College of Medicine. A super-spreader event is defined as one where a single individual infects a large number of people in a single occurrence.
The choir rehearsal is a key example of super-spreading during the ongoing pandemic, but other incidents have occurred—such as on a cruise ship, at a funeral, during an international conference, and in a women’s prison. While it's more difficult to pinpoint a specific person who may have initiated the outbreak in these instances, all of them resulted in a significant number of infections.
While the idea of a super-spreading event may seem terrifying, these events are already accounted for in the general transmission rate. Some infected individuals may cause a large outbreak, like what happened during choir practice, but most others will only infect one or two people. The average transmission rate of a virus, known as its R0, is calculated by this measure. For the coronavirus, R0 is estimated to be between 2.0 and 2.5, meaning each infected person, on average, spreads the virus to 2 to 2.5 others, including any super-spreading incidents.
The risk of transmission increases significantly in spaces with high population density and limited ventilation.
As Kulkarni explains, it's not just the number of people in a confined space, but also how long they stay there that heightens the risk of infection. In the case of the choir practice, a large group of individuals spent hours together in a small room, leading to an increased chance of the virus spreading.
On the Diamond Princess cruise ship, 700 out of 3,711 passengers and crew contracted the virus. This scenario was another example of how being in a shared, confined environment—though large and compartmentalized—led to widespread illness. The same pattern occurred at a women’s prison in Louisiana, where nearly all residents of one dormitory tested positive for COVID-19.
As Kulkarni points out, prolonged close quarters in closed environments increase the likelihood of many people becoming infected. Whether it's due to being exposed to respiratory droplets in a conversation, shaking hands with an infected person, or touching contaminated surfaces, the more time people spend together, the higher the risk of exposure to the virus.
Biogen conference: Two days of socializing, with participants returning to their homes afterward
An important example occurred in late February, when the U.S. had relatively few COVID-19 cases. This was the Biogen conference in Boston, which ultimately contributed to an estimated 99 cases among both the attendees and individuals they later came into contact with.
The outbreaks originating from the conference, where a large group of attendees gathered and interacted over two days, were exacerbated by the fact that many participants returned to their respective communities across the U.S. and globally, unknowingly spreading the virus to others.
As Kulkarni points out, the Diamond Princess cruise, the Louisiana prison, and the Biogen conference are all examples of super-spreading events, despite the fact that the source of the outbreak cannot be traced to just one individual.
Isolation and quarantine measures are proven strategies for reducing the spread of the virus.
The positive outcome? Despite a large number of attendees falling ill during the choir practice, it had minimal effect on the broader community's transmission rates. This was largely due to the swift actions taken by the attendees, who began isolating and quarantining on their own before health authorities could intervene. Actions like hand-washing, mask-wearing, social distancing, quarantining after exposure, and isolating when symptoms appear are effective in controlling the infection rate, even after an event with super-spread potential.
“These remain the most effective practices and strategies we have to prevent further spread,” says Kulkarni.
