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The Cargill beef plant in High River Alta., seen here on April 23, 2020, suffered a coronavirus outbreak that led to 1,500 infections.Jeff McIntosh/The Canadian Press

At CHSLD Vigi Mont-Royal nursing home in Mont-Royal, Que., 374 residents and staff were infected with coronavirus, including 70 who died of COVID-19.

The outbreak at the Cargill meatpacking plant in High River, Alta., led to 1,500 infections.

Paying last respects at Caul’s Funeral Home in St. John’s resulted in 143 cases. The outbreak at Mission Institution, a federal correctional facility in British Columbia, was linked to 132 cases, and another at Alberta’s Kearl Lake oil field led to 100 cases.

More recently, 86 cases have been traced to Full Gospel Outreach Centre in Prince Albert, Sask., 85 to SpinCo, a spinning studio in Hamilton, Ont., 68 to a night of karaoke at Bar Kirouac in Quebec City, and 49 to a wedding in Calgary.

That is just a small sampling of the “superspreader” events that have been recorded in Canada and documented by researchers at the London School of Hygiene & Tropical Medicine.

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All these outbreaks have sporadically made headlines. But after eight months of living with COVID-19 in this country, only now are we starting to focus real attention on superspreading and what it means to controlling the pandemic.

A lot has been learned about the nasty bug SARS-CoV-2 (the virus that causes COVID-19) in a short time, but it continues to surprise and bedevil.

Early on we learned that the coronavirus can be spread by asymptomatic carriers (people who are infected but have no obvious symptoms) and that had a big impact, notably the embrace of masks.

Another intriguing learning is that the novel coronavirus spreads heterogeneously rather than homogeneously; in other words, it spreads in bursts rather than evenly.

That’s why we are seeing clusters of cases (superspreader events) rather than a steady rise in cases across the population, as we tend to see with other viruses such as the flu. In technical scientific language, it overdisperses.

Zeynep Tufekci, an associate professor in the School of Information and Library Science at the University of North Carolina at Chapel Hill, published a fascinating 5,000-word article in The Atlantic magazine on this phenomenon, and its implications for our public health response.

Traditionally, to get a sense of how infectious a virus is, epidemiologists measure the R0 (pronounced R-naught), the average number of infections caused by a single person. If R0 is 2, a carrier infects two others on average.

But averages can be misleading. Currently, R0 is low, hovering just above 1 in much of Canada. But that doesn’t mean each infected person is infecting only one other.

Studies have repeatedly shown that about 80 per cent to 90 per cent of cases are caused by 10 per cent to 20 per cent of carriers. The flip side of this is that most people who get infected with coronavirus won’t infect anyone else.

Epidemiologists are paying more attention to overdispersion, which is measured by k value. Measured on a scale of 0 to 1, the k value of coronavirus is about 0.1, meaning it spreads highly unevenly; by contrast, the pandemic influenza of 1918 had a k value of almost 1, a steady, predictable spread.

As unpredictable as a person’s infectiousness might be, there is no question the environment facilitates spread.

Superspreading events all have certain characteristics in common: They happen in poorly ventilated indoor areas where people congregate for significant periods of time. Weddings, churches, long-term care facilities, crowded workplaces, prisons, gyms, bars, etc., are the hot spots.

Every superspreader event violates the 3C rule: Avoid crowds in close contact in enclosed spaces. And the problem is exacerbated if other measures such as mask-wearing and handwashing are not practiced, or if people are vulnerable, as in nursing homes.

The good news is that understanding overdispersion can help us refine our pandemic response. Clearly, more must be done to limit indoor gatherings and institutional crowding.

We also need to do testing and contact tracing differently.

As Dr. Tufekci notes in her piece: “In an overdispersed regime, identifying transmission events (someone infected someone else) is more important than identifying infected individuals.” That is best done with rapid tests.

Similarly, we need to shift from prospective contact tracing (trying to find contacts once a person is infected), to backward tracing (figuring out who has infected the subject). That way you will identify many more cases, more quickly.

Our pandemic response is geared to tackling a virus spreads in linear fashion, but coronavirus spreads in a more random fashion.

Paradoxically, that means we have to be far more targeted in our response, cracking down hard on large gatherings and environments that facilitate superspreading, and then focus on cluster-busting.

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