Coronavirus and the role of the “super spreader”

How one person can accelerate a pandemic. 

 

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On 10 February, the UK government announced that the coronavirus outbreak, first reported in Wuhan in China on 31 December last year, represented a serious and imminent threat to public health here. The declaration – which means that infected people can now be held in quarantine against their will if deemed necessary – came as Britain’s fourth case of the virus was hospitalised amid fears he may be a “super spreader”. 

The notion of the super spreader came to the fore in 2003 when Sars (severe acute respiratory syndrome), a previous coronavirus pandemic, spread across the world. A Chinese doctor, Liu Jianlun, had been among the first to treat Sars victims in Guangzhou, the capital of Guangdong province, where the virus had transferred from a civet cat to humans. Sars infected more than 8,000 people, killing 774, before being contained in July 2004. 

Dr Liu visited Hong Kong for a wedding and checked into the Metropole Hotel on 21 February 2003. He began to feel ill and by 22 February realised he had Sars and admitted himself to Kwong Wah Hospital. Before leaving the hotel, however, he had transmitted the virus to 17 others, who, before their own symptoms set in, carried the virus to many destinations. Multiple Chinese cities, Singapore, Vietnam and Canada all acquired the virus. Dr Liu died on 4 March and it is believed that around 80 per cent of all of the Sars cases in Hong Kong, where 299 0f the 1,755 infected people died, could be traced directly to him.

Super spreaders are defined as individuals who transmit a disease to significantly more people than the average. All contagious diseases have a basic reproductive number, a statistic we call R0 (“R nought”). The R0 value for Sars was calculated to have been three: on average every infected person went on to infect three more. Hence Dr Liu, having infected at least 17 people, was a super spreader. 

It is too early to calculate the R0 value for the current strain of coronavirus, but estimates place it somewhere between two and four. What makes someone a super spreader isn’t clear. Some individuals may emit more viruses than others. Circumstances such as proximity to an airflow system that enhances the spread of germs may also play a part.

Some are not affected by the germs they carry. This was the case of the best known super spreader, Mary Mallon, an Irish immigrant to the US who carried the bacteria that caused typhoid fever, without suffering from the disease herself. Mallon, later dubbed “Typhoid Mary”, spread the disease to dozens of people in families for whom she worked in the early 1900s, before being traced by a private detective seeking the source of a typhoid outbreak associated with one of her employers. It was proven that she did indeed carry the typhoid-causing bacteria and, under public health legislation, she was isolated in a cottage on North Brother Island near New York, though she aggressively refuted the allegations.

After three years, Mary managed to obtain her release, promising never again to work as a cook. But finding other employment was difficult and she went back to cooking under a pseudonym. At Sloane Maternity hospital in 1915, she triggered another major typhoid outbreak, leaving two dead, and was again captured by the authorities and sent back to North Brother island, where she remained isolated until her death in 1938.

According to the medical historian Judith Leavitt, in her biography of Mallon, racial prejudice played a major role in the plight of this unmarried, working-class Irish woman who spread disease to upstanding American citizens. It’s alarming today to hear of racial slurs aimed at Chinese citizens because of the coronavirus’s origins. 

The response to a possible super spreader in Britain has turned unseemly too. Steve Walsh’s identity was confirmed on 11 February, after pressure from tabloid newspapers and social media. The 53-year-old appears to have contracted the disease in Singapore while at a business conference, before returning and setting off on a skiing holiday in France. It seems that at least five people with whom he shared a chalet picked up the virus and, on his return to Britain, Walsh may have infected two doctors at a GP surgery in Brighton. Though he has made a full recovery, there is concern that others with whom he came into contact, including those sharing a flight back from France, may have been exposed. 

Certainly, if a significant number of infected people are now carrying the virus to different parts of the country we will see a rise in incidence in the UK. However, the health service has been preparing over the past few weeks for such a scenario. And although the global number of fatalities continues to climb (1,108 reported deaths on 11 February) the rise in people reported as infected, compared to the day before, has dropped from 1.4 to 1.1. The UK should be well placed to contain the disease should an outbreak ensue. 

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Michael Barrett is professor of biochemical parasitology at the University of Glasgow

This article appears in the 14 February 2020 issue of the New Statesman, Power without purpose