Coronavirus super-spreaders: Who -- or where -- are they?

As the province settles into phase one of its reopening framework in response to the COVID-19 pandemic, businesses, services and communal spaces in Ontario are beginning to welcome the public once again. Ontario is open for business – at least in part – which means the risk of community transmission of the coronavirus could increase, simply because there are more people in more spaces.

For the first time in several weeks, citizens are able to enter certain retailers, visit their dentist, have their homes cleaned – but is there data to tell us what locations are the most risky in terms of exposure to the virus? Should we be looking at locations, instead of people, as super-spreaders of the disease? Will visiting a crowded supermarket put you at more risk than visiting a clinician’s office?

Paul McDonald

Paul McDonald, dean of the Faculty of Health at York University, says it’s not a simple yes or no.

The term “super-spreader” typically refers to certain individuals who, for reasons that are not fully understood, have a number of biological characteristics that enable them to spread a disease with greater capacity than most people.

“When we talk about super-spreaders as people, they seem to give off more of the virus, so they discharge or shed more of the virus or do so for longer periods of time than other people in the population,” McDonald explained.

So, if a super-spreader can be identified, and then isolated from their networks, it radically improves the ability to prevent transmission. For example, diseases like measles, typhoid fever, tuberculosis and SARS typically spread largely in clusters. As a general rule in the function of disease, it’s said that 20 per cent of people spread 80 per cent of the disease – this is called the “k Factor,” and represents the extent to which a particular infection clusters together.

“What we think is happening with COVID-19 is that it has a particularly high cluster factor,” said McDonald. One estimate, he says, indicates that as many as 10 per cent of people are spreading 80 per cent of the disease.

“So if we know where those clusters are, we can quickly do contact tracing, and in the case of locations, we can close those places or do things like physical distancing among those people in those places,” he said.

Applying the same logic to locations, however, doesn’t translate.

“It’s certainly an interesting idea, and probably comes out of the fact that what we are seeing is about one in every 16 people in long-term care facilities has contracted COVID-19 compared with one in 600 in the general population. So there is a tendency to say that maybe long-term care facilities are ‘super-spreaders,’ but the challenge is that not all long-term care facilities were hit equally, so it’s more likely related to events or activities within a given place than the type of location,” said McDonald.

COVID-19 is thought to have an extremely high cluster factor, with research pointing to as many as 10 per cent of people spreading 80 per cent of the disease

Meat packing plants are another example where one might be inclined to label those facilities as super-spreaders, but McDonald says there are other factors to consider. For instance, meat packing plants require people to work in close proximity and they operate in a cooler environment and, under those circumstances, viruses often linger longer on surfaces which may impact the type of transmission.

Another consideration is ambient noise. McDonald says there have been studies that hypothesize that environments where people are speaking louder, shouting or singing, push out more of the virus, or more droplets of the virus, and increase risk levels. Meat packing plants, he points out, are noisy environments.

“I think there are a series of circumstances, events or activities that combine with some of the physical characteristics of a place that increase the risk, but there is so much variation from place to place, you wouldn’t want to universally say every single meat packing plant should be shut down,” he said. “There are many variables that indicate that one specific location may not be anywhere close to a level of risk than another.”

Public health experts are no doubt taking this into consideration during the reopening framework, says McDonald, but there is also the need to balance risk with necessity; people need access to basic goods and services.

Risk assessment is a very specific practice that should consider an individual’s own situation, the risks associated with specific outings and the necessity of those outings. People should be focusing more on things like density, ventilation and their own health as opposed to a specific type of location.

There's a need to balance risk with necessity when being out in public during the pandemic

“What we know is that the level of transmission in outdoor spaces is typically much lower than indoor spaces (because the air is fresh as opposed to filtered), but outdoor spaces can still pose a risk depending on the behavior of people,” he said.

Groups of people funneling through a gate or using public washrooms, for instance, would increase risk at outdoor locations.

When asked about the risk level of university campuses like York, McDonald said the same type of risk assessment applies. University campuses aren’t necessarily high-risk locations – but do pose higher risks in the same way other high-density spaces would.

“I think universities are fairly low risk overall, but there are certain instances where there may be higher levels of risk. One of the places that public health is very keen to look at is residences. Any time you have large numbers of people in high-density living spaces, it becomes a greater potential risk than, say, sitting six feet apart in the courtyard outside of Vari Hall.”

But York University is not unique. Anytime there is a large group of people converging in a small space – like through a doorway – or touching shared objects such as doorknobs and railings, there is greater risk.

McDonald advises against labeling locations as super-spreaders because it creates negative associations and sparks fear. A relevant example is how emergency rooms in hospitals have reported a 50 per cent decrease in activity since the pandemic because people are afraid to visit hospitals. However, those in medical distress who are delaying visits to emergency could be putting their health at even greater risk.

"That’s why I caution against labeling certain classes of places as being high risk, it brings with it all kinds of unintended consequences – the hospital being the most dramatic example," McDonald said. "But, we don’t want people to stop buying food if somebody suddenly overreacts and labels all grocery stores as a risk.

"It's important for control and better practice to understand where the virus is likely to hit, and protect those who are most vulnerable."

By Ashley Goodfellow Craig, deputy editor, YFile

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