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4 Aug 2020 17:19
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  •   Home > News > International

    Superspreaders and the role they play in transmitting coronavirus to others

    What is a superspreader? Why are they important? And are we seeing many superspreader events in Australia?

    A Melbourne man visits a busy pub in south-west Sydney with several workmates on July 3, and 12 days later 34 COVID-19 infections have been linked to the Crossroads Hotel.

    A woman with no symptoms returns to China from the US and is self-quarantining at home, but after using the lift in her building alone sparks a cluster of 71 cases.

    A choir practice in the US leads to 52 people being infected with the virus and two of the singers dying.

    These are all examples of coronavirus superspreader events, where one or a few infected people spread the virus to many others.

    But how do such events occur? Let's unpack what being a superspreader means.

    What is a superspreader?

    While it's easy to imagine people infected with COVID-19 have equal chance of transmitting the virus to others, it turns out that's not what happens most of the time.

    In fact, the pandemic sweeping the globe seems to follow a pattern seen in many other infectious diseases, where it's been observed that only a small proportion of those infected control the bulk of transmission events.

    This has sometimes been dubbed the 80/20 rule because around 20 per cent of people control around 80 per cent of the infection spread.

    The rest of the time — in fact the majority of the time — the virus doesn't spread from an infected person to anyone else at all, but rather the transmission just peters out.

    Such individuals who infect a disproportionate number of others have been called superspreaders.

    What makes some people superspreaders?

    People can be superspreaders for a variety of reasons.

    These might be biological (they might have particular genes that make them more infectious), behavioural (they might socialise widely, creating more opportunities for infection spread) or simply a random event (they might have been in a crowded room at just the time when they were most infectious).

    Why are superspreaders important?

    Some experts argue superspreading events are of greater interest than superspreaders themselves.

    This is because often there is nothing unusual or unique about the person involved, but rather they were in the right place in the right time to trigger a large chain of infections.

    Labelling someone a superspreader may unfairly stigmatise them.

    There is also the chance some transmissions that were in fact due to other people were missed, making someone look like more of a superspreader than they really were.

    Any individual could be a superspreader given the right circumstances, which is why measures like physical distancing, hand hygiene and getting tested and self-isolating if you have symptoms are crucial for everyone.

    Identifying superspreaders can be useful though because efforts targeted at them could offer more 'bang for buck' in containing outbreaks.

    For instance, contact tracing — which aims to identify other people who an infected person might have passed the virus onto — is often very labour intensive.

    But the effort is more worthwhile for a superspreader than for someone who doesn't pass the virus onto anyone else.

    The trouble is it is often not clear who superspreaders are until they have already infected a large number of people.

    Analysing circumstances in superspreading events however can offer vital lessons that may help prevent or contain other superspreading events.

    Have there been superspreading events in the current COVID-19 pandemic in Australia?

    Yes. As well as the Crossroads Hotel event mentioned above, Professor Mary-Louise McLaws from UNSW Sydney said the recent explosion in transmission in Victoria stemmed from a number of clusters which could be considered superspreading events.

    The clusters involved security guards at quarantine hotels connected to large family gatherings.

    Another cluster emerged in a public housing tower where a combination of the physical environment (communal spaces, cramped living conditions and even ventilation) and employment patterns (many of the residents worked in essential jobs that exposed them to many potentially infectious people) may have created a 'perfect storm' for superspreading.

    Other examples include a wedding at Stanwell Tops in NSW in March when guests newly arrived from the US infected 35 other guests.

    At the Newmarch House nursing home in western Sydney, one infected staff member led to the deaths of 19 residents, and 37 residents and 34 staff testing positive for the virus.

    Speaking on Coronacast today, the ABC's Dr Norman Swan said people need to be careful in pubs, restaurants, gyms and even their own homes.

    "Pubs are potentially superspreading environments," Dr Swan said.

    "In a pub you speak more loudly because you've got to be heard ... and you are drinking therefore you're disinhibited and you're more likely to be laughing, chatting to your mates.

    "It's just the environment where if you've got the virus, it's coming out and it's going significant distances."

    © 2020 ABC Australian Broadcasting Corporation. All rights reserved

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