Analysis: Officials have said there’s no reason to suspect the latest outbreak has involved a ‘super-spreader’ event – but what does that mean? Marc Daalder reports
From a church in South Korea to a college bar in Ingham County, Michigan, sudden coronavirus outbreaks often appear to have one thing in common: a super-spreader event.
Since the Prime Minister announced on August 11 that Covid-19 had returned to New Zealand, officials have taken pains to emphasise that there is no reason to suspect the new outbreak has involved a super-spreader event. Even the Mount Roskill church “mini-cluster” of 21 cases (and counting) hasn’t raised worries among officials and earlier questioning about a church service of concern in Māngere East was dismissed by Director-General of Health Ashley Bloomfield.
Under repeated questions about whether the Māngere service might be a “super-spreader” event, Bloomfield insisted there was no evidence that it was, despite some 300 people attending. The source of this dissonance was a lack of understanding about what exactly super-spreading is.
Super-spreading broadly refers to a scenario where one person infects a large number of others – far above the average reproduction number (sometimes styled as R0) of the disease in question.
A 2005 paper in Nature posited that most infectious disease outbreaks involve super-spreading. Often, some 20 percent of individuals are responsible for as many as 80 percent of cases. This could be due to behaviour of the people, the environment they find themselves in, the disease in question or the physiology of the people – or any combination thereof.
The paper was fostered by public discussion after the original SARS virus displayed a new kind of super-spreading, in which certain individuals appeared to be more infectious than others not by virtue of their actions or even necessarily their environments, but for reasons that were not yet fully understood.
In the 2002-2004 SARS epidemic, super-spreading often coincided with hospital transmission. A fisherman in Guangzhou, where the outbreak began, infected 30 nurses and doctors after checking into hospital on January 31, 2003. A doctor from that same hospital travelled to Hong Kong and stayed in a hotel for one night before being hospitalised. He infected 20 other people in the hotel in the course of his stay, many of whom exported the virus to other countries.
In scenarios like these, the super-spreaders did not engage in behaviour that made it easier for them to spread the virus to others – they were simply more infectious than the average case.
David Murdoch, an expert in infectious diseases and Dean of the University of Otago, Christchurch told Newsroom more about how super-spreading happened for SARS.
“For whatever reason, we may not know, in the secretions that are transmitting to other people there is just more virus,” he said.
“The more virus, the more likely to transmit. We don’t know why it happens. There’s probably something to do with the immune system that allows that to happen. But the end product is the same thing, you’ve got someone who is shedding a whole lot of virus and a lot more than others.”
What about Covid-19?
The similarity of SARS-CoV-2 – the virus that causes the Covid-19 disease – to SARS led to early speculation that super-spreaders could come to define this pandemic as well. Thus far, Murdoch says, there is no evidence that some people may be carrying higher viral loads than others.
“With SARS, there were just those really big examples that did come out, and although it’s been inferred that potentially with Covid, it still is in that theoretical realm. I must admit I’m not aware of really good data to suggest that there is a very clear distinction like there was with SARS,” he said.
But there is evidence of individual people leading to significant numbers of infections. In other words, we know there is super-spreading, even if it isn’t SARS-like super-spreading.
“There seem to be some situations where there was pretty rapid spread and that it was somebody who had increased capacity to do so.”
That’s been visible since the start of the pandemic. A patient in Wuhan in January infected 13 health care workers, in a startling similarity to SARS super-spreading in hospitals. South Korea’s first real outbreak was kickstarted by a super-spreader at a church service in Daegu, which ended up being linked to 544 Covid-19 cases in a matter of days. A week later, the number of infections among church-goers had surpassed 1,000.
After a symptomatic person attended a choir practice in Skagit County, Washington in March, 32 people tested positive for Covid-19 and another 20 were categorised as suspected cases. That left just nine people who weren’t infected at the session. At a college bar in East Lansing, Michigan, 144 people were infected over the course of a single night in June.
Super-spreading in New Zealand
These are super-spreading events – they are events with suitable conditions for transmission of Covid-19 (indoors, close quarters, singing and shouting) attended by a super-spreader. As Bloomfield was getting at back in early August, just because someone with Covid-19 attended a church service does not mean the event will be a super-spreading event. The person would have to be a super-spreader for it to be one.
“We usually reserve the term for when there’s something different about the person, rather than the environment,” Murdoch said.
“The super-spreader doesn’t apply to the event; it applies to the person. So there are some people who seem to shed an enormous amount of virus,” Bloomfield said.
“It’s a very small number of people who seem to shed an enormous amount of virus and create a lot more infection of other people than normally happens. Remembering the sort of R factor of this virus is usually somewhere between three and five people infected by one person, there are some who seem to infect dozens or even hundreds of people.”
While we don’t know exactly how or why super-spreading occurs with Covid-19, it’s clear that it does. It is also looking increasingly likely that the reproduction number for SARS-CoV-2 of two to three is an average figure, but not the median. A comprehensive study of a Hong Kong cluster indicates most people with Covid-19 probably only infect one or no others, while a small proportion of people (10 to 20 percent) lead to the majority of infections (80 percent).
Siouxsie Wiles, a microbiologist at the University of Auckland, suggested studying New Zealand’s own clusters might reveal more information about how and why super-spreading of Covid-19 occurs. Outlining the likely direction of transmission between cases and determining how large the viral load was for each confirmed case could reveal any potential link between large amounts of virus in a given person and the number of infections that person causes.
Murdoch said this was a promising line of inquiry for future research, but added that it would be heavily dependent on the available data. Wiles also noted the data might be skewed if people got tested a while after symptom onset, when they had a lower viral load and well after they had infected others.