What we know about Ontario's September surge in COVID-19 infections

Public health officials can't trace how roughly half of Ontario's latest COVID-19 cases got infected, even as Premier Doug Ford prepares fresh measures to try to slow the pace of spread.

To gain insights into the September surge of COVID-19 in Canada's largest province, CBC News has analyzed Ontario's dataon active cases those who have most recently tested positive for the virus and are either hospitalized or still considered to be infectious.

This gives a clearer picture of current trends that can't always be spotted in the province's daily release of COVID-19 numbers.

Of the more than 2,300 currently active cases in Ontario:

• The suspected method of exposurefor 54 per cent of cases is either unknown, missing or labelled as "no epidemiological link," which means the novel coronavirus isbeing spread in the community.
• More than one-third of active cases are among people in their 20s, even though that age group makes up only 14 per cent of the province's population.
• More than half of active cases are in just two public health units Toronto and Peel Region.

The data suggests that many Ontarians are currently contracting COVID-19 through unmemorable interactionswith others in the course of their daily lives. Experts are worried that failingto track the source of so many new infections will hamper efforts to rein in the spread of the virus.

"If we don't understand how and where people are getting infected, it's very hard to control this disease," said Ashleigh Tuite, epidemiologist at the University of Toronto's Dalla Lana School of Public Health. "It suggests that our contact tracing is not up to the level that we wanted it to be."

Measures to reduce the spread of COVID-19 need to be based on good data about the types of locations and activities that are driving the increase in infections, said Tuite in an interview Wednesday. "If we have a large number of cases who are getting infected and we can't trace where they're getting their infection, it's really hard to respond to that."

Provincial and local officials are poised to announce stricter prevention measures in Ontario's most-affected regions, such as lowering the maximum size of social gatherings and stiffer fines for people who break public health rules.

Currently all of the province's public health units are under Stage 3 of looser pandemic restrictions. Stage 3 allows gatherings of as many as 50 people indoors and 100 people outdoors, with the requirement that physical distancing of two metres be maintained between people not in the same social circle.

CBC's analysis shows that Ontario's active cases the bulk of which have been reported since Sept. 1 are concentrated in the province's most densely populated urban areas. Ottawa and the five public health units in the Greater Toronto Area account for 84 per cent of the current cases.

WATCH | Ontario can't trace half of its COVID-19 cases:

Ontario cant trace half of its COVID-19 cases

4 years ago

Duration 2:03

A study of active COVID-19 cases in Ontario reveals 54 per cent have no known infection source, including a majority that are believed to be the result of community transmission.

"If we act quickly and strongly in those regions that are seeing increased cases,we might be able to avoid wider-spread restrictions," said Tuite.

There are just six active cases in all of northern Ontario, home to nearly 800,000 people.

The vast geographic differences in the infection rate make it unlikely that the Ford government will impose any across-the-board rollbacks of Ontario's reopening plan.

Ford has in recent days spoken with mayors and public health officials in Toronto, Ottawa and the Peel Region cities of Mississauga and Brampton about tighter restrictions. "When we're all agreeing on policies and guidelines to be put in place, we act quickly and we're going to act quicklyon this," Ford told a news conference Wednesday.

As the pandemic has worn on, there's been a noticeable rise in cases among younger adults, and the current Ontario data shows a dramatic surge among people in their 20s.They now make up 34 per cent of Ontario's active cases.

That means someone in their 20s is more than twice as likely to have a fresh case of COVID-19 than you'd expect based on the size of that population.

Nearly two-thirds of all active cases in the province are among people younger than 40, a demographic that represents roughly half the population.

"You hear anecdotally, it's because people in that age bracket are less scared and they're not taking the precautions that other age groups are taking," said Tuite. "Another explanation is that a lot of people in that age demographic need to be out to work.They're the people who are working in restaurants. They're the people who are working in bars."

New information from Toronto Public Health suggests staff are currently far more likely than customers to contract COVID-19 in retail and food-service environments.

Younger people make up bulk of cases

The flip side of thedemographic data suggests there's been some recent success in preventing the spread of the disease to the most vulnerable age group, Ontario's oldest citizens, but there are concerns that may not continue as the pace of spread accelerates.

Another factor in Ontario's September surge that worries the epidemiologists is the rising percentage of Ontario's daily tests that are positive for COVID-19.

"When you start having higher positivity, it suggests that we're probably missing a fair number of cases," explained Tuite.

Research by Tuite and her colleagueDavid Fisman showsthe positive test rate among people in their 20s has shot up in the past few weeks and is currently running above four per cent, roughly four times higher than the general population.

The World Health Organization recommends governments impose tighter public health restrictions if they are seeing a positivity rate of five per cent.