BerandaComputers and TechnologyThe Science of Superspreading

The Science of Superspreading

A fateful conference

In late February, drug company Biogen held its annual conference
in Boston. The United States had fewer than 20 known COVID-19
cases at the time. But one of the roughly 200 attendees must
have carried the virus.

It triggered a big outbreak. At least 97 people who attended the
conference, or lived in a household with someone who did, tested

The Biogen meeting had become a superspreading event.
Eventually, the virus spread from the meeting across
Massachusetts and to other states.
A recent study
estimates it led to tens of thousands of cases in the Boston
area alone.


COVID-19 superspreading events have been reported around the
world. They happen in all sorts of places: bars and barbecues,
gyms and factories, schools and churches, and on ships.

And even at the White House.

But why do these disease clusters occur—and why are they so

The reproduction rate

COVID-19 and many other diseases transmit from person to person.
The reproduction rate, R, determines how fast a disease
can spread.

R denotes the number of people infected, on average, by a single
infected person. If R is 2, the number of cases doubles in every
generation: from one infected person

to four, to eight, and so on.

What happens in the real world

Real life isn’t that neat.

One person may infect four people.

Two of those may not transmit the disease to anyone else. One
might pass it on to three others, and the fourth one may
infect 21 people.

Of those 21, many may not pass the virus on to anyone else. But
perhaps one will pass it on to 18 others.

A few people infect many others

In COVID-19 and many infectious diseases, most people don’t
infect anyone else. A small percentage of people cause most of
the transmission.

How strong this pattern is depends on the disease, but superspreading appears to be particularly important in COVID-19.
A study published in April
estimated that 10% of patients are
responsible for 80% of the spread.

A March wedding in Jordan was another tragic example of the
dangers of superspreading. The father of the bride was infected;
he passed the virus on to at least 76 of about 360 guests,
a study says. One woman died.

Stopping superspreading is key

If a small minority of cases leads to most of the spread, then
stopping these people from transmitting should bring the
pandemic under control. But how?

One key strategy is to eliminate the conditions that favor
superspreading. Researchers have identified several factors that
make superspreading events more likely, known in Japan as the
three Cs: closed spaces with poor ventilation,
crowds, and close contact settings. Shouting,
singing, and heavy breathing also seem to increase the risk of

What it means for contact tracing

Paying attention to superspreading events is also important for
contact tracing.

If you find out someone has COVID-19, you can trace their
contacts and test or quarantine them. But there is a high
likelihood they didn’t infect a single other person.

However, there is a high likelihood that person became infected
at a superspreading event.That’s why contact tracers in some
countries now spend more time understanding where someone was
infected and finding other people from the same cluster.

This is called backward contact tracing. It can help find
more chains of transmission.

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