Who could have guessed choirs could be so dangerous? It's suspected that simply talking can transmit Covid-19. But singing, it seems, is a projectile weapon.
Covid-19 doesn't meet all the criteria to be classified as an airborne disease.
But it gets close.
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Coughing and sneezing certainly shoot small droplets loaded with virus into the air, where they can linger for some time. But there's been a lingering suspicion that Covid-19 can hitch a ride on humid particles expelled through talking.
Pump up the volume, and it seems Covid-19 aerosols can be projected over large distances – especially in an enclosed space.
Such is the implication of a study by the US Centres for Disease Control (CDC).
Its researchers were attempting to further understand the highly contagious nature of the virus. So they drilled-down into the circumstances behind one early infection hotspot.
It was a choir practice held in Skagit Valley, Washington, on March 10 – three days before the White House declared a national emergency.
At this point, the town had no known cases. But one of the choir members had developed cold-like symptoms a few days earlier. But it wasn't severe enough to put them off attending.
What happened next has become a textbook example of a superspreader event.
"Medical advice is to avoid singing in proximity until further research has been done or the pandemic is controlled," says senior lecturer in Voice and Stagecraft Narelle Yeo of the Sydney Conservatorium of Music.
Choir practice turns out to be an ideal study group for epidemiologists.
In this case, there was supposed to be 122 participants.
But news of the pandemic had people concerned. So only 61 turned up.
What happened next gave researchers an ideal opportunity to examine the physics of contagion.
The singers arrived to find six rows of 20 chairs evenly spaced before them. Some chose to sit as far apart from the rest as possible. Others simply took up their familiar positions, regardless of how many were around them.
Researchers identified where each person sat in relation to the carrier.
The session started with a 40-minute mass practice. Then the singers split into two focus groups for a 50-minute intensive. One group moved into a smaller room where they sat on benches. The other was in the larger room but had pulled their chairs into a tight cluster.
After a 15-minute free-ranging break, they all reconvened for a final 45-minute massed practice.
A day later, one person developed symptoms. Within 12 days, 52 had contracted the disease.
Three were admitted to hospital. Two died roughly a week later.
The CDC researchers conclude the experience of the choir reinforces the value of social distancing. In this case, the virus was almost certainly carried by droplets projected through the air. Touching contaminated surfaces was also a risk factor.
This was determined by mapping the known carrier's position within the neatly ranked choir, as well as in the breakaway group.
The proximity of new infections correlated strongly to where this singer was positioned, and where they had been.
"This outbreak of Covid-19 with a high secondary attack rate indicates that Sars-Cov-2 might be highly transmissible in certain settings, including group singing events," the study finds.
Singing raises the threat. But so do certain singers.
"Choir practice attendees had multiple opportunities for droplet transmission from close contact or fomite transmission, and the act of singing itself might have contributed to Sars-Cov-2 transmission," the study reads.
"Aerosol emission during speech has been correlated with loudness of vocalisation, and certain persons, who release an order of magnitude more particles than their peers, have been referred to as superemitters and have been hypothesised to contribute to superspeading events."
In other words, some people project more virus-carrying moisture into the air than others.
"This underscores the importance of physical distancing, including maintaining at least six feet (1.8m) between persons, avoiding group gatherings and crowded places, and wearing cloth face coverings in public settings where other social distancing measures are difficult to maintain during this pandemic."
The Skagit Valley incident wasn't the only superspreader event to involve choirs. A 130-person choir in the Netherlands performed together on March 8. Soon after, 102 fell ill. Four died.
In Germany, a rehearsal at the Berlin Protestant cathedral choir sickened 59 out of its 78 singers. Public singing there is now restricted.
Across Europe and the United States, churches make regular appearances on Covid-19 hotspot lists. Especially the fundamentalist protestant and evangelical churches that have defied distancing measures to conduct their exuberant singing and shouting services.
"Singers inhale more air and exhale at greater air and moisture volume than speakers," Narelle Yeo wrote. "Singing produces six times the amount of airborne droplet nuclei (aerosolised virus particles which can remain suspended in the air) compared with speech."
The University of Sydney lecturer says safe singing in choirs and groups is unlikely to be viable as a safe distance for such an activity is yet to be determined. And there are unlikely to be many venues that would meet such a standard anyway.
This means alternative outlets will need to be found, at least until a vaccine is widely available,
Online practice can be 'hilarious and frustrating', Yeo says, because of transmission lag. But it can be overcome in the spirit of keeping groups together.
"Brilliant visual creations of multiple singers can hide this," she says. "The best choir videos are edited together to synch performances and give the appearance of ensemble singing."