What superspreader events like the one at Hamilton’s SpinCo can tell us about the way forward in the pandemic

At a Hamilton spin studio, a patient zero" with no symptoms started a chain reaction that led to at least 85 infections.

In Toronto, an adult recreational hockey league was linked to more than 21 cases.

And at the White House, a Rose Garden ceremony that included an indoor portion and lots of hugging, kissing and handshakes is suspected to be behind perhaps the most high-profile outbreak of the year.

Superspreader events, where many are exposed to COVID-19 and then pass the virus on to their families, workplaces and schools, have made headlines for major roles in the continued spread of the coronavirus.

There's a growing consensus that not everyone with the disease will infect the same number of others, unlike, for example, the flu. Instead, under the right conditions, a few will infect a lot. Recognizing this overdispersion (measured by something called K) and containing these events are crucial parts of managing the spread, say experts. But these pieces of the puzzle have not been central to public health messaging, and our personal risk evaluations.

The fact that it's overdispersed is both a blessing and a curse, I'd say," said Ashleigh Tuite, an epidemiologist with the University of Toronto's Dalla Lana School of Public Health.

The effective reproductive number, Rt, or the average number of people each infected person will in turn infect, fluctuates given public health measures. It's one indicator experts, including Toronto Public Health officials, track to monitor spread in the community.

But looking at K can also give important clues. Influenza's K value is high, meaning, there's not a lot of variability, and for the virus that causes COVID-19, SARS-COV-2, it's thought to be quite low," Tuite said. And so what that means is there are a small number of people who are disproportionately responsible for a large number of cases."

One paper that looked at clusters in Hong Kong found 19 per cent of cases seeded 80 per cent of all local transmission.

Another massive study of almost 85,000 cases in India found five per cent of people accounted for 80 per cent of the infections identified through contact tracing.

And there have been multiple reports of events, such as a choir practice in Washington state, where one or two people infected dozens.

It would be possible to calculate K locally, in theory, but not without very good contact tracing data, because you'd have to find out how many people each person infected, Tuite added. More feasible is recognizing the unique behaviour of the virus, and adapting policies.

COVID spreads primarily through respiratory droplets, so if people are close together inside, without masks, there's a risk. Smaller droplets can linger in the air, particularly when there's poor ventilation. It also depends on how contagious a person is, said Colin Furness, an infection control epidemiologist at the University of Toronto, and they can pass it on even if they don't have symptoms.

If we're putting people in the same room with no masks, we are possibly creating the conditions for a superspreader event," he said. But it's hard to predict which fitness class, bar or family dinner will have that perfect storm" of people together for prolonged periods, and someone happens to be maximally contagious."

That's what appears to have happened in Hamilton at a SpinCo studio that's become one of the biggest superspreader events in Canada, now linked to 80 infections. The index patient, the Star's sister paper the Hamilton Spectator reported, had no symptoms. The studio screened everyone coming in for signs of illness, and they kept bikes six feet apart.

One way that public health can help stem the exposures from superspreader events is through backwards contact tracing," said Tuite. Instead of just looking at an infected person and trying to reach all close contacts so they can isolate and get tested, ideally health units should also try to trace back positive cases, to see who infected them, and potentially identify other cases from a large transmission event.

That's the approach taken by some places that have contained COVID. In Japan, a small army of contact tracers, in place long ago as part of a response to threats like tuberculosis, has focused on cluster busting," finding and containing everyone associated with a cluster, or event, and shutting it down before it gets out of control.

Hamilton Public Health Services said it did use this backward tracing for SpinCo, a common tracing method" it uses to help with outbreak tracing," spokesperson Jacqueline Durlov said in an email. It did not respond to requests for more information on this, citing privacy concerns.

But in Toronto, with public health overwhelmed by community transmission, there's no contact tracing now, except for institutional outbreaks in places such as schools, hospitals and long-term-care homes, a shift to prioritize high-risk settings, TPH spokesperson and associate medical officer of health Dr. Vinita Dubey, said in an email.

Earlier this month the province closed gyms, bars, indoor dining, fitness centres, theatres, bingos and casinos. Wedding receptions were limited to 10 people inside and 25 outside in hot spots Toronto, Ottawa, York and Peel for 28 days. That followed weeks of warnings from doctors and Toronto's public health boss Dr. Eileen de Villa that these places were especially dangerous. Private gatherings are also limited to 10 people inside and 25 outside, province-wide.

Cases are now so high in Toronto that the risk of encountering the virus at smaller gatherings has shot up and de Villa is telling people not to gather at all inside with members of other households and to limit trips to essential outings. Social bubbles are also paused across the province, in response to rising infections.

Dance studios in hot zones were allowed to reopen Tuesday, with officials saying they were able to safely distance.

To help overloaded local health units, the province has hired 100 contact tracers, with 500 more expected on board by mid-November, said health ministry spokesperson Christian Hasse in an email, as part of a $1.376 billion plan to enhance and expand testing and contact tracing. Over 200 staff are going to Toronto, where public health says there are currently 700 tracers.

But Dubey said given the resurgence in cases, even doubling the number of cases and contact staff would still likely not be sufficient to contact trace with the same reach and results as when infection rates were lower."

Even a few superspreader events can overwhelm a health unit's contact tracers, said Tuite, which is where Toronto is now.

When you have too many cases and you can't do that thorough and thoughtful contact tracing, then the focus really just needs to be on getting cases back down to a manageable level," she said. So you basically have a bit of a reset, and moving forward you can focus again on that thorough contact tracing," in a bit of a different way."

We're learning, unfortunately through trial and error," she added, and it's clear, for example that the policies for gyms reopening were not sufficient. Moving forward we need to revise those policies, to either focus on ventilation and making sure that people are wearing masks, and not just when they are getting to and from their activity."

Furness believes venues with the conditions for superspreading such as bars, indoor dining and gyms need to shut down province-wide, and the ones in hot spots need to remain closed, especially heading into winter, when dry inside air helps the virus spread.

The Hamilton spin studio, he notes, did everything right and still had a massive outbreak.

We created the situation where superspreader events can then happen," he said. Having these venues open is also telegraphing to people" that it is OK to gather inside in groups, and inviting them to have their own parties, he said.

In March and April, we didn't understand how the disease spread and that's why the message was pretty brutal in terms of be afraid, don't touch anything, don't do anything." Now that we know more, we should be focusing on superspreading," he said. we desperately need to change our public health messaging."

We ought to be mostly able to do the things we want to do. There's very few things we can't do."

As for what individuals can do to prevent these events, it's clear that gathering in large groups is unacceptable," said Dr. Barry Pakes, an assistant professor and program director, public health and preventive medicine residency program at the Dalla Lana school.

Still, he worries about Halloween. The province has advised against trick-or-treating - but large adult parties are a real danger now.

Lots of people outside your household, indoors, when you might be consuming alcohol, or wearing costumes where, in general on Halloween people do things they wouldn't otherwise do," he said. That's where we really need to be careful."

With files from the Hamilton Spectator

May Warren is a Toronto-based breaking news reporter for the Star. Follow her on Twitter: @maywarren11