Toronto neighborhood COVID-19 risk analysis shows “step 2 backtracking was not effective”


With new COVID-19 infections in Ontario now averaging more than 900 per day, new data analysis shows your chances of encountering a person infected with the virus in groups of 50 are now over 25% in 42 Toronto neighborhoods, compared to 16 just three weeks. since.

The analysis, carried out by Ryan Imgrund, biostatistician at the Southlake Regional Health Center, also shows that the odds of coming into contact with the novel coronavirus in this size group are over 50% – one in two – in four neighborhoods in the city.

The data, which provides figures on the risk in the neighborhood for the period of October 14 to 25, also shows that the probability of encountering a person infected with the virus in groups of 100 was 25% or more in 84 neighborhoods, compared with 66 three weeks. since.

“I think this shows that so far, going back from step 2 has not been effective,” said Imgrund, who is also a high school science teacher and worked for the Agence de la public health of Canada. “I expected to see a plateau for the worst. Instead, we saw a bigger increase than expected given the return to the modified Stage 2. “

However, at a press conference Friday to announce millions of dollars in funding for two Etobicoke schools, Premier Doug Ford said he had asked public health experts to come up with a plan this week. next to “ease restrictions so as to safely allow businesses to start back at the end of the 28 day period.” “

The Prime Minister said: “Based on the evidence I see, the steps we had to take (October 10) have had an effect.”

The amended Stage 2 measures included banning indoor dining and fitness classes in gyms, as well as closing cinemas and limiting gatherings to 10 people indoors and 25 people outside. ‘outside. Toronto, Peel and Ottawa were the first to enter the amended restrictions, followed a week later by York Region.

At Friday’s press conference, Dr Barbara Yaffe, the province’s deputy chief medical officer of health, said that before the amended Stage 2 restrictions, about 30-40% of COVID-19 cases in Toronto and Ottawa came from bars to restaurants. That proportion has now fallen to around 14% in Toronto, Yaffe said.

Overall, cases continue to increase, but “they’re progressing more slowly,” she said. “So what that tells us is that maybe we can consider changing the conditions.”

Currently in Toronto, Imgrund calculates that the overall risk of encountering someone with COVID-19 in a group of 50 is 19.1 percent. For groups of 100 people in Toronto as a whole, your chances of encountering someone infected with the virus are 34.5%; in a group of 500, the risk increases to 87.9%; in a group of 1000, the risk is 98.5%.

The province paused the 10-person social bubbles in early October, when it imposed masks in indoor spaces, when the physical distance of two meters is not possible, across the province. .

To calculate the risk percentages, Imgrund uses postal code case data released by the City of Toronto on a weekly basis. It assumes the virus is transmissible for a period of 12 days and takes into account that people can transmit COVID-19 before and after symptoms appear.

It also takes into account the population of each neighborhood and uses seroprevalence figures from Public Health Ontario, which tests specific antibodies to COVID-19 in blood samples to determine the proportion of the population that has been infected.

“What I’m looking at here are the cases that can be passed on to other people,” he said. “There are a few sophisticated math tricks that need to be done to account for data cleansing, reporting delays and future case growth, but these are just sophisticated math tricks that use the data already available that Toronto is releasing. . ”

Imgrund noted that his risk calculations represent a snapshot in time going back 12 days. This means that the neighborhoods in which people are more likely to meet someone with COVID-19 this week may not be the same next week.

Chris Bauch, professor of applied mathematics at the University of Waterloo, points out that while the data calculates the likelihood that a person could encounter someone with COVID-19 in their neighborhood, it is not a model of transmission , and it is not. show a person’s chances of being infected by this encounter.



He said that in neighborhoods where the risk of encounter is higher according to Imgrund’s calculations, individuals can reduce their risk of transmission if they are “diligent about mask use, removal physical, hand washing and staying in small (social) bubbles that don’t have limbs that are also part of other bubbles. “

“The smaller the bubble, the better,” Bauch said.

But he warned: “If cases start to climb (as they will), the chances of meeting someone, even in small groups, will start to increase.”


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