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Coronavirus: Why ‘superspreaders’ may make COVID-19 easier to control

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SARS-CoV-2, the coronavirus that causes COVID-19, in an electron microscope image made available by the U.S. National Institutes of Health in February 2020
SARS-CoV-2, the coronavirus that causes COVID-19, in an electron microscope image made available by the U.S. National Institutes of Health in February 2020   -   Copyright  NIAID-RML via AP
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There is scattered but growing evidence that a small number of so-called "superspreading" events are responsible for a majority of coronavirus infections.

To the layman, the idea may sound frightening, implying that the virus can easily spiral out of control and infect a whole crowd.

But for some epidemiologists, it’s good news.

"If we can stop those larger outbreaks, they can really do a lot of good in terms of slowing down transmission without needing to resort to lockdowns. We just need to stop superspreading," Benjamin Cowling, a professor of public health at Hong Kong University, told Euronews in a live TV interview.

He and a team of scientists recently published a preprint paper – preliminary, still to be peer-reviewed – examining 1,038 cases of COVID-19 in Hong Kong between January 23 and April 28 which, using contact-tracing, identified all local clusters of infection.

In the study, just 20 per cent of cases, all of them involving social gatherings, accounted for 80 per cent of transmissions.

Equally surprising was that 70 per cent of the people infected did not pass on the virus to anyone, Cowling and the study’s lead author noted in an op-ed in the New York Times.

The findings have yet to be scientifically reviewed, and given the experiment took place in Hong Kong, where the total number of infections has been relatively small (at just over 1,000 cases), it may not be representative of the way the virus has spread across other countries with bigger outbreaks.

Other studies, however, have pointed in the same direction. One, published in The Lancet in April, studied 391 cases of the new coronavirus and more than 1,200 of their close contacts in Shenzhen, southern China. It estimated that 80 per cent of transmissions of the virus had been caused by just 8 to 9 per cent of cases.

Similarly, mathematical modelling led by Akira Endo, of the London School of Hygiene and Tropical Medicine, estimated that "80 per cent of secondary transmissions may have been caused by a small fraction of infectious individuals (around 10 per cent)."

So what defines a 'superspreader' – and how can we avoid them?

"With COVID-19, on average, one person might infect two others – and that's what was happening in the early stages of the pandemic," Cowling explained.

"Superspreading is the idea that not everybody infects two others. Some cases might not infect anybody at all. And others might infect quite a lot of others."

Those are the infamous "superspreaders" – a small fraction of infected patients spreading the virus to a larger chunk of the population. Of course, these people may well have no symptoms, no idea they are contagious, and be fully unaware they are infecting others.

Some people are simply more contagious than others, but that in itself is not enough to define a "superspreader", Cowling said. The context also appears to be very important.

"We also need the environment, maybe a crowded setting, maybe indoor with lots of people, maybe poor ventilation, lots of close contact, lots of time there," he said, adding that almost all of the superspreading events his team examined have occurred indoors.

Cowling hailed Japan as an example in fighting superspreading events, with a strategy known as avoiding the "three Cs": closed spaces, crowds, and close contacts.

He believes that social distancing measures will need to remain in place for at least the next six months, but that authorities should figure out ways to allow gatherings and special events such as weddings and funerals to go ahead safely.

"Outdoors would be a good idea," he said, suggesting there should also be fewer people gathered in larger venues.