Where do people get COVID-19? Community transmission remains a puzzle in Ontario


Nine months after the start of the COVID-19 pandemic, health officials still face a conundrum as vexing as at the start of the epidemic: determining how cases spread in the community.

In Ontario, up to 60% of cases have unknown sources of transmission.

The lack of data has, in part, led to our current lockdown, as government advisers look to the United States for how far it takes to go to shut down parts of the economy.

“The restaurants did everything they were asked to do and more. Public servants make decisions based on theories and assumptions. But the financial and reputational damage to this industry is not theoretical. They’re all too real, ”said James Rilett, a spokesperson for Restaurants Canada, a non-profit association that advocates for the food service industry.

Frustration over the province’s decision to send Toronto and the Region of Peel to lockdown has sparked backlash from business owners this week, with some opening up to contempt for health measures and others calling for the closure of big box stores considered essential services.

One of the reasons for the lack of data on where the community has spread is that it is extremely difficult to find contacts of strangers in crowded places like restaurants, bars and gyms. The growing number of cases in recent weeks has made contact tracing in some areas, such as Toronto and Peel, nearly impossible. (In early October, Toronto Public Health announced it was suspending contact tracing outside of high-risk situations and institutional outbreaks due to the growing number of cases in the general community.)

But researchers at the COVID-19 Science Advisory Table, a group of scientists and health policy experts who analyze trends and advise the provincial government, say there is enough evidence from U.S. studies and local epidemic data to make educated guesses about where the virus is most likely. spread.

“It is highly likely that much of what goes undetected in statistics and in contact tracing, no matter how best people are doing, is happening in the indoor spaces where people meet most. Said Dr Peter Juni, Scientific Director of the Science Advisory Table. and professor of medicine and epidemiology at the University of Toronto.

“Indoor gatherings where people are relatively close to each other – it can be you and your friends; it doesn’t have to be other people you don’t know – and where people stay for an extended period of time, i.e. more than 15 minutes, that’s actually what happens with meals inside.

In a report earlier this month, the advisory board linked the frequency of visits to restaurants with indoor dining and the potential for the virus to spread if those establishments reopen.

The report also shows that while less than 10% of COVID-19 cases in Ontario are linked to outbreaks, restaurants, bars, ceremonies and religious services account for a total of 9% of these outbreaks. The settings most affected by the outbreaks are long-term care homes and retirement homes (26.5%) and schools and daycares (22.2%).

“This does not mean that the proportion shown in these figures for restaurants covers all those infected in a restaurant; it only covers those we know, ”said Beate Sander, co-chair of the Ontario COVID-19 Modeling Consensus Table. “This high proportion of community transmission occurs in schools, in restaurants and in gyms and in workplaces and… everywhere, basically.

A snapshot of data from Toronto Public Health shows that for a week in September, there were 45 active outbreaks in the workplace, 18 of them in restaurants and bars, one in a nightclub, and another in an adult entertainment club.

“Together, these outbreaks alone accounted for 44% of our community outbreak investigations,” said Dr. Vinita Dubey, Toronto Assistant Medical Officer of Health, in an email, adding that the investigations “were extremely resource-intensive.”

“Bars and restaurants have a large number of contacts to trace,” she said, “some of these establishments having more than 500 contacts to notify, and one with 1,700 customers to reach.”

The province closed indoor restaurants, bars, gyms and theaters in Toronto, Peel and Ottawa on October 10. Gyms were allowed to reopen earlier this month before being closed again in Toronto and Peel under the new lockdown, which also closed outdoor restaurants, hair salons and non-essential retail .

Dubey said the strategy behind the current lockdown is to limit the ability of people to spend time together in high-risk environments where the risk of infection is increased.

“It’s a difficult choice, but a necessary one right now,” she said.

When asked if there was evidence of closing restaurant patios, as Toronto and Peel did this week, the Department of Health said in an email the lockdown was needed to limit transmission community in order to “keep schools open, protect the capacity of the health system. and protect the province’s most vulnerable populations.

“In partnership with the Chief Medical Officer of Health and our local medical officers of health, we are continuing to closely monitor developments to indicate if and when public health measures need to be adjusted,” a spokesperson for the Department said. ministry.

A September U.S. Centers for Disease Control and Prevention study found adults confirmed to have COVID-19 were about twice as likely to have reported eating at a restaurant in the two weeks before their illness compared to those who tested negative.

Another American study, published this month in the British science journal Nature and supported by the Ontario Science Advisory Table, suggests a link between people’s destination and infection patterns.

The study used anonymous cellular data to track the movement of 98 million people in major US cities from neighborhoods to hundreds of thousands of what they called “points of interest,” which included restaurants, grocery stores and churches, March through May.

The researchers ran a simulation to predict, based on visits, where residents were infected and found that a small fraction of points of interest accounted for a large percentage of infections. The simulation results produced infection rates close to what was happening in cities at the time.

These points were typically places that “tend to have denser visitors and visitors tend to stay longer,” such as restaurants, said Emma Pierson, who co-authored the study while working on her doctorate. at Stanford University. Pierson holds an MA in Statistics from the University of Oxford, where she was a Rhodes Scholar, and is now a Senior Researcher at Microsoft.

Density was also key to why the model predicted higher infection rates in neighborhoods with fewer whites, which is consistent with what actually happened in the first wave, Pierson said.

The model showed that people in these neighborhoods were not able to reduce their mobility as much, in part because they were more likely to be essential workers.



“It’s hard for them to work from home,” Pierson said.

The places they went generally had more visitors and were “therefore more dangerous,” Pierson said, noting that in the United States there is a correlation between higher population density and lower socio-economic areas. .

Although the study in Nature did not assess whether individual restaurants were riskier than other points of interest, a simulation to reopen each area by category, while keeping the others closed, showed that the reopening of restaurants would cause hundreds of thousands of infections in the Chicago area. .

Reopening full-service restaurants was particularly risky, the study found, with researchers predicting about 600,000 more infections in the Chicago area by the end of May, more than triple the next category of establishment.

Pierson said part of the reason could simply be the number of restaurants.

In the metropolitan areas used in the study, full-service restaurants, as well as gyms, hotels, cafes, religious organizations, and limited-service restaurants, produced the largest increases in infections predicted during reopening.

Statistics from the Nature study used by the Ontario Science Advisory Board show that additional COVID-19 infections associated with reopening sites (after a lockdown, for example) are much higher for the top 10%. poorer employees than for the 10 cent.

The risk of becoming infected in restaurants, places of worship and grocery stores was disproportionately high for the poorest 10%.

“These are the essential workers. These are not the people who buy their organic bananas. It’s the people who work there who are infected, ”Juni said. “It’s like the perfect storm for transmission. They are exposed because they are essential workers. “

Further research in the United States could shed light on how retail businesses and restaurants can reopen more securely in the event of a pandemic.

Nature’s study found that if Chicago’s points of interest capped maximum occupancy at 20%, it would reduce new infections by more than 80% while reducing the total number of visits by just 40%.

“If you reduce the maximum occupancy, it can potentially allow you to reopen more efficiently because it creates a strong reduction in infections for a relatively small reduction in visits,” Pierson said. “The reason is that it only affects the places where they are most congested and dangerous. “

The authors also believe that the results of the study show that the reopening has a disparate impact on at-risk neighborhoods, a risk that could be mitigated by strict occupancy caps, the distribution of food in areas with high-rise stores. risk, more testing and an improvement in paid leave or income. supports for essential workers as well as personal protective equipment.

Toronto had a higher incidence of the disease in the northeast and northwest parts of the city.

Juni said he hopes the science advisory table will find ways to make restaurants and other places safer.

“Basically try to wear a mask as often as you can, try to ventilate as much as possible in those places,” he says. “Keep the tables separate from each other so that at least groups of people don’t interact with each other. All of this will be useful in the future. “

“The problem is, only if you restrict (yourself) relatively heavily will you get this thing under control because it’s so contagious.”


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