In a new study, researchers found that singing indoors, unmasked can swiftly spread COVID-19 via microscopic airborne particles known as aerosols.
They examined a March choir rehearsal which became one of the nation’s first super spreading events.
This study documents in great detail that the only plausible explanation for this super spreading event was transmission by aerosols.
The research was conducted by a team at the University of Colorado at Boulder.
On March 10 in Skagit Valley, Washington, one person with mild symptoms of COVID-19 attended a 2.5-hour choir practice indoors.
In the weeks that followed, more than 50 other people from that rehearsal would contract the disease—almost everyone who attended—and two died.
Because participants had taken precautions to sanitize and avoid touching each other, scientists suspected that aerosol transmission, not larger drops spit into the air or infected surfaces, were the culprit.
The new study confirms it.
Members of the chorale were serious about their music and their health that day. They did not touch each other, touched a few shared surfaces, propped doors open, and used hand sanitizer.
Few people shared the same restroom as the infected person, and many who did not use any restroom got sick.
They did not, however, wear masks.
The team interviewed the chorale through a representative about what happened that day and calculating the rate of infection based on the details of the rehearsal and what is known about the SARS-CoV-2 virus.
They concluded that there simply were not enough opportunities for droplets and infected surfaces, known as fomites, to transmit the virus to the number of people who fell ill afterward.
But poor ventilation in the indoor space led to a build-up of aerosols produced by the singers, and heat produced by the singers themselves mixed the air within the room.
There were also many singers present and the rehearsal was long.
The researchers found that shortening the rehearsal time in the Skagit Valley event from 2.5 hours to 30 minutes would have dropped the rate of infection from 87% to 12%.
Wearing masks, improving ventilation, using portable air cleaners, and rehearsing for half of the duration combined could have dropped the number of people infected down from 52 to only 5.
This super spreading event happened early on in the pandemic when there were no known cases in Skagit County.
At the time, businesses were just starting to shut down in the U.S. and public health officials were just beginning to debate whether masks were necessary.
The new study offers new insight into how those outbreaks occurred and what can be done to make future choral rehearsals safer.
The authors recommend conducting choral practices outdoors whenever possible during the COVID-19 pandemic, and carefully managing any indoor singing events, as singing can generate large amounts of the aerosolized virus if any of the singers is infected.
Improved ventilation which draws in more outside air, and air cleaning, that removes virus-containing aerosols from the air, can be helpful to reducing the spread of airborne infections in any indoor space, but singing with masks and at distances more than 6 feet apart is very important as well.
One author of the study is Shelly Miller, a professor of mechanical engineering.
The study is published in Indoor Air.
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