The risk of coming into contact with COVID-19 has increased in 96 Toronto neighbourhoods over the past week and is highest in two neighbourhoods that border each other in the city’s northwest corner.
In a crowd of 50, the chance of encounterning someone with COVID in the Thistletown-Beaumond Heights neighbourhood surrounding Islington Ave. and Albion Rd. was nearly 25 per cent; in a crowd of 100, it was more than 40 per cent, according to a tool that analyzes recent case data by postal code.
The risk was similar in the Rexdale-Kipling neighbourhood, which borders Thistletown on its southwest side. It’s the second week in a row that Thistletown-Beaumond Heights has had the highest risk.
In comparison, the current overall risk in Toronto is much lower — 6.4 per cent in a group of 50 and 12.4 per cent in a group of 100, according to Ryan Imgrund, a biostatistician with Southlake Regional Health Centre, who did the analysis. Imgrund’s analysis presents risk by neighbourhood and size of potential gatherings of individuals.
Toronto has been a hotspot for the virus in recent weeks and on Thursday, was one of three cities where the province decreased the size of allowable gatherings, reducing the maximum of 50 for inside gatherings to 10 and for outside gatherings from 100 to 25.
Next week’s analysis of risk should show if the new size limits have had a positive affect.
“Parents and school staff need to make sure they are using the government’s COVID-19 screening tool every morning. Make it part of your routine if you haven’t already,” said Imgrund, who is also a high school science teacher and has worked for the Public Health Agency of Canada.
“If your child has access to a phone, download COVID Alert on it to alert them in a timely manner of any possible school exposure. If you haven’t reduced the size of your at-home bubble, now’s the time to do it – particularly if you have older individuals inside of it.”
Imgrund has been posting daily updates on Twitter showing the virus’s daily reproduction number in the province, as well as a slew of other data, and says he started calculating risk as a way to give some context to the daily case numbers that we’ve been inundated with since the start of the pandemic.
“What this tool does is … gives you a perspective of the risk in your neighbourhood,” said Imgrund. “It will allow parents to decide, based on the numbers in their neighbourhood right now, ‘Is the risk of my child encountering COVID-19 acceptable? Am I comfortable with them returning to school knowing the risk of COVID-19 in my neighbourhood?’”
“We’ve become almost immune to those numbers,” he said. “I started doing a risk analysis because I thought it was more important to give context to (them).”
Dionne Aleman, an associate professor at the University of Toronto and an expert in pandemic modelling, says the presence of the virus in the community could have an impact on what happens in schools.
“So much is unknown about how COVID is transmitted among kids and from kids to adults,” she said.
But she said there are no borders on neighbourhoods, and the presence of the virus anywhere in the city, even in the Greater Toronto Area, is a concern.
“Any city is sufficiently interconnected that people frequently interact with others from other neighbourhoods,” said Aleman. “For example, say the Annex has zero cases, and Koreatown (which is adjacent to the Annex) has 200 cases; it would be madness to say that schools are safe to open in the Annex.”
To calculate the risk percentages, Imgrund uses case data by postal code released by the city of Toronto every week. He assumes that the virus is transmissible for a period of 12 days and takes into account the fact that people can transmit COVID-19 before and after the onset of symptoms.
He also accounts for the population of each neighbourhood and uses seroprevalence figures from Public Health Ontario, which is testing for antibodies specific to COVID-19 in blood samples to determine the proportion of the population that has been infected.
“What I’m looking at here is the cases that are able to be transmitted to other people,” he said. “There are some fancy math tricks that need to be done to account for data cleaning, reporting lags and also future growth of cases, but they’re just really fancy math tricks that are utilizing the data that is already out there that Toronto releases.”
If a neighbourhood hasn’t recorded any cases in the past 12 days, no risk calculation can be made, he added.
Imgrund noted that his risk calculations represent a snapshot in time going back 12 days. That means neighbourhoods in which people have higher odds of encountering someone with COVID-19 this week may not be the same next week.
Chris Bauch, a professor of applied mathematics at the University of Waterloo, points out that, although the data calculates the probability that someone could encounter a person with COVID in their neighbourhood, it is not a transmission model, and it doesn’t show a person’s chances of becoming infected through that encounter.
He says that, in neighbourhoods where the risk of an encounter is higher according to Imgrund’s calculations, individuals can reduce their chance of transmission if they are “diligent about mask use, physical distancing, handwashing and remaining in small (social) bubbles that don’t have members who are also parts of other bubbles.”
“The smaller the bubble, the better,” Bauch said.
But he cautioned that “if cases start to climb (as they will), the chances of encountering someone, even in smaller groups, will start to increase.”