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A skiing trip, a wedding, a choir practice: what these events have in common is that they were all occasions of coronavirus “superspreading”. This is when someone passes the virus on to an especially high number of people.
While there is no universally agreed definition of a superspreading event, it is sometimes taken to be an incident in which someone passes on the virus to six or more other people. Getting to the bottom of why these puzzling clusters occur could be key to gaining control of the covid-19 pandemic and stopping a second wave of cases.
For months, we have been hearing that the R number, or reproduction number, is what is needed to gauge the spread of covid-19. This is the average number of people that each infected person passes the virus on to. Before lockdown in the UK, the R number for coronavirus was estimated at somewhere between 2 and 3.
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It is now more appreciated that there is great variability in the number of new cases that each infected person generates. This can be described by the epidemic’s “K number” – the dispersion parameter – with a lower value of K signifying more variability. You need to know both R and K for a good picture of how the virus is spreading through a community.
According to an analysis of how covid-19 had spread to other countries from China by the end of February, the K number was 0.1, an extremely low value. The researchers estimated that 80 per cent of cases were caused by about 10 per cent of infected people. Those 10 per cent could trigger a cluster of infections, while most other people would pass on coronavirus to no one else and a few would give it to just one other person.
In other words, superspreading is integral to the pandemic, says Quentin Leclerc at the London School of Hygiene and Tropical Medicine.
In one well-studied example, at a choir practice with 61 attendees that took place one evening in March, in Skagit County, Washington State, one person infected an estimated 52 others. Doctors followed up with the close contacts of every secondary case, about three or four each, and could find only 10 further infections, says Lea Hamner, a public health official in Skagit County. It is as if something qualitatively different were going on that night.
What might that be? Transmission clusters have been seen with other diseases, including HIV, TB and typhoid, with a famous superspreader being a New York cook in the early 20th century who came to be known as Typhoid Mary. In these cases, it seemed there was something biological that made the person more likely to pass on their germs, probably a heavy microbe burden.
With coronavirus, virus burden may well play a role, but this hasn’t been investigated and we have no easy way to start doing so, says Benjamin Cowling at the University of Hong Kong in China. “If we measure viral load in saliva, that’s not the same as how much virus they’re breathing out. You would have to do some kind of air sampling.”
But as well as biology, the circumstances of the spreading event also seems to be important, and some common themes have emerged. Cowling’s group carried out contact tracing of the first 1037 coronavirus cases in Hong Kong. They found a somewhat larger K value than the previous estimate, of 0.45, but that still means just 20 per cent of infected individuals caused 80 per cent of locally acquired cases.
The team found that superspreading events tended to happen in indoor spaces, with people in close proximity. Social occasions led to more clusters than exposure in the workplace or home – mass transmissions occurred at weddings, temples, bars and karaoke parties, for instance. The risk seems to be higher if people are raising their voices in some way, such as singing or shouting. “It’s the volume of air that comes out of your lungs,” says Cowling.
Understanding superspreading is becoming even more important now that coronavirus cases are declining in many countries, says Adam Kleczkowski at the University of Strathclyde in the UK. When case numbers are rising exponentially, superspreading events are arguably less important, he says, as there are many clusters that spread and merge.
But avoiding these transmission clusters is key early in an epidemic before case numbers have risen sharply, or later on, when cases are falling and a second wave must be avoided. “When you have very few cases, it’s these [superspreading] events you need to watch out for,” says Leclerc.
How can we reduce the chances of further superspreading events? “Our guiding principles right now are outdoors is safer than inside, fewer people are safer than more people,” says Hamner. She sees indoor bars as a particular risk. “A drunk person is well known for talking louder and louder the drunker they get.”
These aren’t new ideas – but making people aware of how important superspreading is could reinforce the need to avoid or at least mitigate these types of risky situations. It could also help inform how countries ease lockdown restrictions, says Leclerc, whose team has created a database of superspreading events around the world. For instance, only eight of the 201 clusters they identified took place within schools, which at the start of the pandemic were seen as a potential hotbed of infections.
Identifying clusters in real time is also key for contact-tracing work, says Leclerc. “If you manage to detect [superspreading events] and find the people infected, you can stop the spread before it goes on.”
Reference: Research Square, DOI: 10.21203/rs.3.rs-29548/v1
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