February 20, 2020

expert comments on what a ‘superspreading’ event is

There have been reports of a ‘superspreading’ event in the outbreak of COVID-19 in South Korea.

Dr Ben Killingley, Consultant in Acute Medicine (Clinical Lead) and Infectious Diseases, University College London Hospital, said:

“The contagiousness of infection is usually defined in terms of the reproductive number – the average number of people infected by one diseased person.  This number can range from between 1-2 for seasonal influenza up to about 16 for measles.  For SARS it was estimated to be about 3 and COVID19 so far seems similar to this.

“It has been well recognised for many Infectious Diseases that some individuals seem to be implicated in spreading infection to many more people than would be expected, hence the term super-spreaders.  Spread or transmission of infection depends on several variables related to the infected individual, the environment and the recipient of infection.  For example, an infected person with prominent symptoms e.g. cough, who has a high amount of virus in the respiratory tract (viral load) and continues to mix socially might be the perfect vector.  The perfect environment to help spread infection might be an enclosed space with no ventilation where people get close to each other.  A prone recipient might be someone who has a weak immune system or other medical conditions that increase their chances of becoming infected.

“At face value it does seem as though the incident in South Korea was a super-spreading event, however, we need to be cautious until more information comes to light.  We would need to confirm whether there really was only 1 individual at the centre of this incident (could there have been more unrecognised cases?) and that others didn’t get infected in other environments outside of the church.

“So, superspreading events like this are determined by a collection of circumstances that come together to produce the right conditions for spread – patient, environment and new hosts.”

Dr Bharat Pankhania, of the University of Exeter Medical School, said:

“The loose and non-scientific term “Super Spreader” is a misnomer.  There is no such thing as a super spreader.  What we have are circumstances that lead to the infection of a larger number of people.  These circumstances often are: crowding; a confined space with poor ventilation; poor infection control, meaning lots of non-porous, hard surfaces which can keep a virus viable for a longer time; a favourable ambient humidity; and the infected person usually being in the early phase of their illness, when virus secretions are at their peak.”

Prof Rowland Kao, Royal (Dick) School of Veterinary Studies and Roslin Institute, University of Edinburgh, said:

“A superspreader will be anyone who is able to infect an unusually high number of other individuals, when under normal circumstances.  Being a superspreader could be due to a person shedding unusually high quantities of virus, or engaging in behaviour resulting in substantial potentially infectious contacts.

“It looks from the news reports that this woman was in a situation which might lead to unusual levels of contact with others – it’s important that the contact tracing be done to determine if the high number of infections can be explained by that behaviour.

“If that is the case, while technically we may have seen a superspreading event, it would not be the type of event that presents a worry to the general populace (since it is unlikely that any of them would experience contact in a similar way).  However, like the Diamond Princess situation, it does highlight that, under the right circumstances, Covid-19 does have the potential to infect many individuals.”

Prof Christl Donnelly, Professor of Applied Statistics, University of Oxford, and WHO Collaborating Centre for Infectious Disease Modelling, Imperial College London, said:

“Disease transmission is heterogeneous.  It is highly variable.  R₀ is the population average for the number of secondary cases caused by a single infected individual in a fully susceptible population.  If this average is 2, for example, we would still expect that some people would infect 0, some 1, some 2 and so on but the average is 2.

“By simple chance, we’d expect the distribution to look like this.

“So in this case 99.55% of people would transmit to 6 or fewer people.

“There is no clear line as to what might be superspreading.  10 or more perhaps?  We’d expect only 5 in 100,000 people to transmit to that many just by chance, if everything else were equal.

“But we are not all the same, we vary in our immune systems, in our behaviour, and in where we happen to be.  All of these things can affect how many people we would transmit to.  Thus, biological and behavioural factors can contribute, but so can time and place.  In Amoy Gardens during the SARS epidemic in Hong Kong, there didn’t appear to be anything unusual about the patient that visited that high-rise building, but the conditions in the building were such that a super-spreading event occurred and over 100 people became infected with SARS.  There has been debate about whether within Amoy Gardens it was airborne spread, faults in the sewage system or both, but there remains no suggestion that it was the patient that was unusually infectious or exhibited any unusual behaviours.

“There is a push for using the terms “superspreading” and “superspreading event” rather than “superspreader” which by virtue of labelling the person implies that the person was to blame.”

All our previous output on this subject can be seen at this weblink:

http://www.sciencemediacentre.org/tag/covid-19

The SMC also produced a Factsheet on COVID-19 which is available here:

https://www.sciencemediacentre.org/smc-novel-coronavirus-factsheet/

Declared interests

None received.