Marco Bottigelli via Getty Images / WIRED
When 61 members of the Skagit Valley Chorale choir turned up to their weekly rehearsal on March 10, the coronavirus pandemic seemed like a distant threat. In the whole of Skagit County – a slice of northern Washington State with a population of 129,000 – there were just two confirmed cases of the illness. For two-and-a-half hours the choristers practised their songs, stopping part way through to snack on cookies and oranges and discuss the concert they had scheduled for that April. Although they didn’t know it at the time, this would be the last practice that Skagit Valley Chorale would have for some time: one of the members in attendance was already infected with coronavirus.
Two weeks later, 53 of the attendees had reported symptoms of the viral infection – an attack rate of 87 per cent. Of those, 33 tested positive for Covid-19 and a further 20 were considered to have probable infections. Three of the attendees had such severe cases that they were hospitalised. Two of those patients later died from the infection. That one choir practice, where the average age of an attendee was 69 years, had become the epicentre of a Covid-19 outbreak.
In the weeks before she received the first report of the choir group outbreak, Lea Hamner, Skagit County Public Health’s lead for communicable disease, had watched the reports of a super-spreading event linked to a church in South Korea. “That was the only super-spreader event that I was aware of at the time, but nothing in the US had ever happened like that by March 17. I did not expect what we found.” On that day, Hamner’s team received a call from a chorister saying that three members of the group had positive Covid-19 tests and a further 25 were showing symptoms. Suddenly, Hamner was dealing with a super-spreading event of her own.
Although coronavirus has been sweeping the globe for five-and-half months, we are only just beginning to piece together an understanding of how the disease spreads. We still don’t know, for example, what role people with no symptoms play in transmitting the disease, or how long people stay infectious for. But we are beginning to understand that Sars-CoV-2 does not spread evenly across the population – there is increasing evidence that some of the infected spread the virus much more widely than most. Understanding this phenomenon – known as super-spreading – will be vital to managing Covid-19 as countries enter the difficult no-man’s-land where the disease is neither peaking, nor entirely eliminated.
The Skagit County choir outbreak was a “tragic, tragic, situation” says Hamner. But it is far from exceptional. Other super-spreading events have been catalogued at barbecues, meatpacking plants and funerals. In a preprint scientific paper – yet to be reviewed by other scientists – researchers at the University of Hong Kong found that 20 per cent of Covid-19 cases accounted for 80 per cent of all transmissions. In one cluster identified in the study, 73 infections were linked to one individual attending bars in Lan Kwai Fong – a small nightlife spot in central Hong Kong.
The study of super-spreading events turns the public understanding of how coronavirus spreads on its head. We’re used to seeing charts that show how one person who contracts the virus can pass it on to two others, who each pass it on to two more, on and on leading to an exponential increase of cases. But this simplifies how transmission actually occurs. In the Hong Kong example, “rather than one person spreading to one or two others, then them spreading to one or two others [and so on], it was most likely one person spreading to a lot of other people in one go,” says Ben Cowling, a professor at Hong Kong’s School of Public Health and an author on the pre-print study.
In the UK, the government has placed a lot of emphasis on the R value of coronavirus: a number that captures how many people, on average, each person infected with Covid-19 passes the disease on to. At the start of the epidemic, the R0 (the transmission rate when no measures to curtail the spread have been taken) was thought to be between two and three. In the UK, after months of lockdown the R is between 0.7 and one although in parts of the country the number may have ticked back over one.
But to really understand the spread of the disease, we might be better off looking at another number: the K value. Unlike R, K tells us how transmission numbers vary across a given population. In simple terms, a low K value suggests that a small number of cases are responsible for large amounts of disease transmission. It’s important to know this number because knowing how a disease is spreading might change the kind of health interventions we put in place to stop it.
When a disease is still circulating widely in a population, as has been the case in the UK for several months now, pinpointing super-spreading events is tricky. England only resumed contact tracing at the very end of May, after suspending it on March 12 for 11 weeks. In Hong Kong, which has a total of 1,108 coronavirus cases and just four deaths, it’s much easier to get a clear picture of how the virus spread. Cowling’s study found that out of the 349 locally-transmitted cases they identified (the majority of the region’s cases were imported from outside) 196 of them were linked to just six superspreading events.
But what turns certain situations into super-spreading events? The Skagit Country and Hong Kong examples share some qualities that made transmission more likely. In both situations, a large number of people spent an extended period of time in very close proximity. The chairs at the Skagit Valley Chorale practice session were spaced just six-to-ten inches apart and the session lasted two-and-a-half hours. (The Centers for Disease Control and Prevention defines ‘close contact’ as being within two metres of someone with Covid-19).
Although much of the press coverage around similar events has concentrated on the people carrying Covid-19 (so-called ‘super-spreaders’), Megan Murray, an epidemiologist at Harvard University says that we should by paying much closer attention to the kind of environments in which super-spreading events happen. Epidemiologists talk of the “three Cs” that make transmission more likely. “Those are closed spaces, meaning buildings with poor ventilation or rooms with poor ventilation, close contact, so lots of people in those small spaces, and then crowds,” Murray says.
This means that governments looking to halt the resurgence of Covid-19 should be looking for ways to prevent these kinds of situations. Rather than rushing to reopen bars and restaurants, they should slowly expand the number of contacts between people in a way that avoids indoor locations where lots of people are going to be present, she says. In the UK, the government is reportedly weighing up whether to relax the two-metre social distancing rule that currently makes it difficult for pubs and restaurants to find a way to safely re-open.
In the summer, where people have access to outdoor spaces, that might be less of an issue, but people crowding into poorly-ventilated pubs in the winter months could create the perfect environment for super-spreading events. William Ristenpart, a professor of chemical engineering at the University of California, Davis, studies how humans release microscopic particles of fluid during speech. He authored a 2019 study detailing that the louder someone talked, the more particles they released as they spoke, which could mean that loud environments, such as bars, pose an extra level of risk for transmission.
But proving this conclusively for Covid-19 is tricky. At the moment, most of the evidence suggests that the Sars-CoV-2 virus is mainly transmitted through droplets – large particles emitted when people sneeze or cough. There is limited evidence that Sars-CoV-2 is also present in aerosolised particles – very small particles that float in air for much longer periods of time and can potentially infect people over greater distances, although this is unlikely to be a common route of transmission.
In either case, Ristenpart recommends that increasing ventilation in indoor environments could reduce the risk of them turning into super-spreading locations. “I would urge people to think about increasing rates of ventilation,” he says. “There are energy costs associated with that [but] that seems like, to me, a small price to pay for helping decrease the probability of transmission.”
If a super-spreading event does happen, then it will come down to effective contact tracing to find the people who may have been in that environment and urge them to isolate. If contact tracers can find and isolate potentially infectious people fast enough, then it dramatically reduces the risk that those people will then become part of another super-spreading event.
In the end, that may have been what stopped the Skagit County outbreak from spreading much further than the choir members and their very close contacts. By the time Hamner’s team had contacted all of the choir members, almost all of them had begun to isolate themselves anyway, she says. “We pretty much did not see many cases outside of that choir group and their immediate family,” she says.
Since the outbreak, the choir members haven’t had another practice but they have attended a webinar with infectious disease experts who spoke about why singing might increase the risk of transmitting viruses. Hamner is hopeful that other groups will find a way to adapt their events until things return to normal. “There’s a lot of engineering we can do to still have the essence of our events while adapting,” she says. “I will always be advocating for adaptations and smarter versions of people’s events until we have a vaccine and widespread immunity.”
Matt Reynolds is WIRED’s science editor. He tweets from @mattsreynolds1
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