9 Percent of 'Superspreaders' Responsible For 80 Percent of Covid Infections

Hint: being indoors with lots of other people talking loudly, singing, dancing, eating and/or sweating is not where you want to be, assuming you'd prefer not to catch the virus. JL

Paul Nuki reports in The Telegraph:

A small number of “superspreading” events appear to be responsible for the great majority of coronavirus cases, raising the prospect of the virus being controlled if those events can be pinned down. By applying a mathematical model to outbreaks of the disease outside China, they estimated that 80% of all secondary transmissions were caused by around 10% infected individuals. Hospitals, nursing homes, dormitories, food processing plants, church choirs, indoor gyms, exercise studios and food markets have all been associated with major outbreaks of Covid-19.

A small number of so-called “superspreading” events appear to be responsible for the great majority of coronavirus cases, raising the prospect of the virus being controlled if those events can be reliably pinned down.

Many infectious diseases follow an “20/80” rule, whereby the majority of cases are caused by a small number of infectious individuals. These include pathogens such as HIV, measles and Ebola, as well as the coronaviruses Mers and Sars. 

As the journal Nature noted recently, “population estimates of R0 can obscure considerable individual variation in infectiousness”.

This is now thought to be the case with Covid-19.

An analysis by researchers at the London School of Hygiene and Tropical Medicine and the Alan Turing Institute strongly suggests there is a “high degree of individual-level variation” in the transmission of Covid-19. 

By applying a mathematical model to reported outbreaks of the disease outside China, they estimated that 80 per cent of all secondary transmissions were caused by a small fraction of infected individuals - around 10 percent.

“Our finding of a highly-overdispersed offspring distribution highlights a potential benefit to focusing intervention efforts on superspreading”, the study concluded.

“As most infected individuals do not contribute to the expansion of an epidemic, the effective reproduction number could be drastically reduced by preventing relatively rare superspreading events”.

The race is now on to pinpoint and characterise these “superspreader” events. If we know where the trouble lies we can let the rest of society open up again.

Tempting though it may be, most experts say we should not look for individuals. Superspreading events are determined by a complex mix of behavioural and environmental factors. 

Even sexually transmitted viruses like HIV tend to be “superspread” more by things like needle sharing and prostitution than individuals. Funerals were a major problem in the 2014-16 Ebola outbreak in West Africa. 

With Sars-Cov-2, it seems likely any infected individual could become a superspreader. Who we are is likely to be less important than where we go and what we do when we are there.

Already, many superspreading venues are known. Hospitals, nursing homes, large dormitories, food processing plants and food markets have all been associated with major outbreaks of Covid-19.

Last week it was reported that four out of five traders (79 per cent) at Lima’s wholesale fruit market in Peru have tested positive for coronavirus, for example. In other large markets across the city at least half were found to be carrying the virus.

Indoor gyms and exercise studios also appear to lend themselves to superspreading events. A new South Korean study found that 112 people were infected over 24 days after attending “dance classes set to Latin rhythms” at 12 indoor sports facilities.

“Intense physical exercise in densely populated sports facilities could increase risk for infection”, said the authors. “Vigorous exercise in confined spaces should be minimised during outbreaks”.   

Just over half of the cases were the result of transmission from instructors to those attending the dance classes and the overall attack rate was a high 26.3 percent.

Characteristics that may have led to the outbreak included “large class sizes, small spaces, and the intensity of the workouts”, said the study.

“The moist, warm atmosphere in a sports facility coupled with turbulent air flow generated by intense physical exercise can cause more dense transmission of isolated droplets”, it noted.

The researchers did not find any cases where classes were limited to five people or less. Also, pilates and yoga appeared to pose a lesser risk than dance.

“We hypothesise that the lower intensity of pilates and yoga did not cause the same transmission effects as those of the more intense fitness dance classes,” said the authors.

But you don’t have to be dancing to be exhaling vigorously while in the close contact of others.

In Washington State on the west coast of America, a church choir went ahead with its weekly rehearsal in early March even as Covid-19 was sweeping through Seattle, an hour to the south. Dozens of its members went on to catch the virus and two died.

The Washington singers were not the only choristers to be hit. Fifty members of the Berlin Cathedral Choir contracted the virus after a March rehearsal, and in England many members of the Voices of Yorkshire choir came down with a Covid-like disease earlier this year. 

A choir in Amsterdam also fell victim to the virus, with 102 of its 130 members becoming infected after a performance. One died, as did three of the chorister's partners.

Research suggests it is not the singing alone that causes the spread of the virus but the close contact that goes with it. 

“These outbreaks among choir members all occurred during the early days of the Covid-19 pandemic, before lockdowns were imposed and before our minds were concentrated on the importance of social distancing”, Professor Christian Kähler of the Military University, Munich, told the Guardian newspaper. 

“Choir members probably greeted each other with hugs, and shared drinks during breaks and talked closely with each other. That social behaviour was the real cause of these outbreaks, I believe.”

One of the biggest superspreading events in Europe came in the February half term holidays when thousands of people gathered in alpine ski resorts. 

Hundreds of infections in Germany, Iceland, Norway, Denmark and Britain have been traced back to the resort of Ischgl in the Tyrolean Alps. Many had visited the Kitzloch, a bar known for its après-ski parties. 

The bar is tightly packed and famous for "beer pong" – a drinking game in which revellers take turns to spit the same ping-pong ball into a beer glass. 

Earlier this year The Telegraph obtained a video from inside the Kitzloch. It may yet come to define the perfect superspreader event, with attendees all singing along to AC/DC’s Highway to Hell.