Coronavirus: What is a super-spreader and how do they change disease outbreaks?

Sophie Gallagher

The coronavirus outbreak has spread across Europe with Germany, Spain, Austria, Switzerland and Croatia all reporting their first cases of the deadly disease.

Matt Hancock, the health secretary, has told MPs that 7,132 people in the UK have been tested for coronavirus; 13 of these have tested positive.

A leaked government document suggests that “the reasonable worst case” scenario would see up to half a million people could due from the disease if it is to infect up to 80 per cent of the country.

In Italy, a dozen towns are in lockdown after 11 people have been killed by Covid-19, but authorities have yet to track down the source of the virus, which they believe might be the result of a so-called super-spreader.

This pattern of transmission from one individual to many others is unusual; so far coronavirus has only seen an average of two or three people infected for every patient.

So why do some people spread viruses to a greater number of people than others, what makes them a super-spreader, and is this a useful medical term or just a way to demonise an individual for something arguably out of their control?

What is a super-spreader?

Although not strictly defined by the World Health Organisation (WHO), a super-spreader is generally taken to refer to a patient who infects significantly more people with a disease than usual.

So far with coronavirus we have seen an average of two or three people infected for every one patient. But because it is only an average, of course, some will infect more and others less.

Mark Woolhouse, professor of infectious disease epidemiology at the University of Edinburgh, told The Independent: “With a super-spreader you are just seeing the tail end of that [average] distribution pattern.”

This means that with diseases other than coronavirus the definition of a super-spreader could be different because the average number of people infected for every one patient is different (for example, during the 2002 Sars outbreak a person had to infect at least eight people to qualify as a super-spreader).

What makes a super-spreader?

Established for at least two decades, the 20/80 rule suggests that around one in five people in the population are likely to be responsible for 80 per cent of transmissions, and therefore a super-spreader. Professor Woolhouse reiterates they are not the norm.

Professor Woolhouse says: “We do not know why some people are super-spreaders, the medical question is whether this is just about chance and a particularly unlucky sequence of events or there is something different about this person.”

He says there are two possible theories about how super-spreaders start. “One reason could be that some individuals shed large amounts of virus so they are more likely to pass this on.

“This type of person is also likely to be sicker so more likely to be hospitalised and therefore presents a risk about hospital transmission.”

However, Prof Brendan Wren, professor of microbial pathogenesis at the London School of Hygiene and Tropical Medicine, says there is no clear evidence this is happening yet with coronavirus. ​

“The other theory,” says Prof Woolhouse, “is that these super-spreaders cope better with the virus and don’t show any symptoms so they are going about as normal and don’t know they have it.”

He says that it is not possible for super-spreaders to know this about themselves and there are likely multiple factors at play.

Of course, coming into contact with more people – either through the course of your job or by travelling regularly, for example, in crowded places or on long-haul flights – could also make you more likely to transmit the virus than those who are more isolated.

Dr Stephen Griffin, associate professor at the University of Leeds, says: “The issue with the term super-spreader is that it implies that the person in question is inherently more able to pass on disease compared with others.

“In fact, it must be remembered that the spread of a virus like coronavirus depends upon a number of factors that ultimately comprise a ‘transmission event’. This will depend upon the patient and what stage of disease they are in, their behaviour, their environment, and of course time.”

Should we use the term super-spreader?

WHO has not officially used the term super-spreader to describe patients, and Public Health England has also confirmed to The Independent they are not using it.

Professor Woolhouse says it can be useful in some contexts but we have to be careful about how it is applied. “Although I recognise the term super-spreader in terms of epidemiology, I would not describe a person as a super-spreader, I would say it is a ‘super-spreader event’ instead.

“I would describe this case history [the man in Brighton], for example, as a super-spreader event because the reason this has happened is a number of things, including the individual, the virus, the contact history – a mixture of many factors coming together.

“There is no context for saying that this person has done anything wrong. We don’t know the underlying biology of this virus yet.”

Dr Sylvie Briand, director of pandemic and epidemic disease at WHO, agrees and is concerned that individuals labelled as super-spreaders will be stigmatised. “We need to talk about super-spreading events and not people. [It is] the circumstances and situation that makes transmission exist, not the person itself.”

Where else have we seen super-spreader events?

Super-spreader events are a feature of nearly every viral outbreak worldwide and are an important part of disease spread, especially in the early stages when it needs to take hold in the human population.

They have been documented as far back as the 1900s when one woman, Mary Mallon, infected 51 people with typhoid even though she had no symptoms herself.

In the 1995 Ebola outbreak in the Democratic Republic of Congo, two people were thought to have infected around 50 others. In the 2014 outbreak of the same virus, it was well documented that a funeral was thought to be the starting place for 100 new chains of transmission.

In the 2002-2003 Sars epidemic, there were super-spreaders in Singapore who infected 10 people each.

One student at a high school in Finland infected 22 others with measles in 1998 even though eight of them had been vaccinated.

Does this change how we deal with coronavirus?

Public Health England says that the emergence of a super-spreader in the UK has not changed the strategy they are using, reiterating the importance of early detection, self-reporting and self-isolation.

“It would be nice if, in due course, we understood more [about super-spreaders] and could then target them during outbreaks,”says Professor Woolhouse. “But for now it doesn’t change how we handle this, just reinforces the importance of the current strategy.”

In the latest statement from the Royal College of GPs, Professor Martin Marshall says: “The current threat to the UK is still considered moderate. Patients should not be alarmed as it is still more likely that anyone with flu-like symptoms will have the flu.

“However, in light of the members of staff who tested positive for coronavirus in Brighton, we’d like to remind the public that it is vital that any patient who thinks they may have symptoms does not try to attend a GP appointment or hospital emergency departments in person.

“They should stay at home and call NHS 111, advising the call handler if they have recently travelled back from one of the following places: China, Thailand, Japan, Republic of Korea, Hong Kong, Taiwan, Singapore, Malaysia and/or Macau.”

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