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Hope in the science of 'superspreaders'

June 3, 2020

The fact that only a few highly infectious people are responsible for the spread of the novel coronavirus sounds dramatic. But knowing that may also make it easier to design a targeted strategy to handle the pandemic.

Berlin nightlife
Image: picture-alliance/dpa/S. Kembowski

The more scientists study SARS-CoV-2, the more we learn about the novel coronavirus and its paths of infection. That is fundamentally good, because it means the pandemic can be handled in a far more targeted manner. But at the same time, that means we may come to question numerous measures that were implemented at the start of the pandemic and which now may appear to have been the opposite: untargeted or perhaps even unnecessary.

Some of the research on "superspreaders" and "superspreader events," for instance, may well lead to such discussions. People may ask: What was helpful, what worked, and what measures will work in future?

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How do you become a superspreader?

Epidemiologists describe superspreaders as people who infect a significantly high number of other people. That's not strictly a superspreader's fault — anyone can become a superspreader if they happen to come into contact with a lot of people at the wrong time.

And the timing is important. A person can be infectious before they see the first symptoms. That's when, it seems, the viral load is especially high in a person's nose and throat.

Infographic - coronavirus exponential growth

But many people have few or no symptoms at all. They often don't realize that they are infected and, as a result, that they are infectious.

In addition, it seems that some people spread more of a virus — and that for a longer period of time — than other people. That may have something to do with their individual immune systems, or the distribution of virus receptors in their bodies.

How do superspreader events occur?

If a highly infectious superspreader comes into contact with lots of other people, it is possible that they will infect a higher than average number of people in a short amount of time, in a closed area.

A high-rise, block of flats in the German city of Göttingen, where scores of people got infected with the coronavirus
When infections spiked after private parties, 700 people in this block of flats in Göttingen/ Germany were put in quarantineImage: picture-alliance/dpa/S. Pförtner

As a rule, it's been possible to trace such "infection clusters" around the world.

In Germany, recent superspreader events include an explosive rise in infection cases after large family gatherings in Göttingen, church services in Frankfurt, and carnival celebrations in Heinsberg.

Similar cases have emerged in clubs, a dance schools in Seoul, South Korea, a bar for skiers in Ischgl, Austria, cruise ships and slaughterhouses.

An airborne transmission?

Studies of superspreader events have shown that particular factors increase the chance of an event taking place. For instance, the risk of infection is significantly higher in closed spaces than outdoors. And the more the people, the greater the risk.

But why is that?

Increasingly, the indications suggest that this aggressive SARS-CoV-2 virus is not only transmitted via droplets and surfaces, but also by airborne transmission — the virus can linger in small and badly ventilated spaces in aerosol form.

A church service in Charleston USA
Communal singing brings the community together. It may also help spread the coronavirus.Image: Reuters/D. Goldman

The studies so far have also shown that the aerosol is quickly distributed via loud talking or shouting — such as you might hear at a bar, club or a sports event. The same is true for singing in church services or in choirs. And some people release more aerosol than others.

Calculating the risk

It's usually the number of new infections that determine what measures to counter the spread of the coronavirus are implemented and to what extent.

One decisive factor is the reproduction rate R — that's the number of people an infected person infects on average. A rate of 2 means that an infected person infects a further two people. So, the aim of social distancing, for instance, is to get the rate as low as possible, and keep it there, down below 1.

Next to the reproduction rate, however, is the so-called dispersion factor k. That number indicates how often an illness occurs and where potential clusters appear. As with the rate of R, it's better to have a low value for k. That would indicate that the potential for distribution is small and the infection can be traced back to a few people or even a single person.

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But scientists have yet to work out the dispersion factor k for the novel coronavirus.

In one well-read study, researchers at the London School of Hygiene and Tropical Medicine (LSHTM) — Akira Endo, Adam Kucharski and Sebastian Funk — conclude that the k-rate of the coronavirus could be 0.1, which would suggest that 10% of infected people are responsible for 80% of all infections.

The LSHTM study is a "preprint" and has yet to be peer-reviewed by the scientific community — it needs to be verified and checked. But its results do coincide with those from other studies, including research led by Christian Drosten, head virologist at The Charité, a university hospital in Berlin. Drosten's research suggests that 20% of infectious people may be responsible for 80% of infections.

What does this mean for the current coronavirus measures?

When the novel coronavirus broke out, scientists around the world came under growing pressure to gather together all the available information. And based on what the scientists said, politicians around the world came under just as much pressure to decide how they could or should best protect people from the pandemic.

The debate about face masks, which were derided before they became compulsory, is one example of a policy that may have to be adapted as new knowledge is won — even if not everyone understands or wants to understand why.

Research into superspreaders so far shows that most infected people infect only few other people, but that a few infected people have the potential to infect many more people. That is good news as it suggests the measures to contain the pandemic can be targeted better than before.

While it's remains difficult to identify and isolate superspreaders, because many superspreaders have few or no symptoms, it is possible to monitor the sorts of circumstances that can create a superspreading event.

One man sits in an otherwise empty sports stadium
A ghostly emptiness at sports events instead of cheers and hugging peopleImage: picture-alliance/dpa/U. Deck

It may be possible to contain the further spread of the virus with ongoing bans on large gatherings, especially in closed environments, and if people follow the other hygiene and distancing rules.

If that works, whole cities won't necessarily have to be closed or locked down, with all the associated economic and social fears that come with that. Perhaps distancing rules could be relaxed step-by-step to allow a relatively normal situation for businesses, schools and kindergartens until a medicinal drug or a vaccine are developed.

But what if that's not possible?

Politicians and other decision makers could easily continue to ban large gatherings, communal singing, and place restrictions on restaurants and cafes. 

Social distancing at a park in San Francisco — everyone stays in marked circles
Not everyone sticks to social distancing rules as well as these people in San FranciscoImage: picture-alliance/dpa/J. Chiu

But it all comes down to how we behavior as individuals. As soon as the weather turns again, and all those parties or meetings outdoors get moved back indoors, there will be plenty of situations, where we will fail to observe those distancing and hygiene rules. And if there's a single superspreader in your gathering, you could have a superspreader event on your hands. 

If that happens, it will be very important to find the new cluster without delay and to immediately isolate everyone who had contact in that situation. Only once that's done, should those contact people be tested — experts say that strategy helps limit the spread of the virus and saves valuable time. 

Japan, for instance, used this method to slowly but successfully contain the spread of the coronavirus and spare the country a full lockdown.