Ontario is reporting more than 500 new COVID-19 cases today, the highest single-day total reported on a Monday since late September.

Provincial health officials logged 552 new COVID-19 infections today, down from 666 on Sunday but up from 480 one week ago.

The seven-day rolling average of new cases is now 573, up from 476 last Monday.

Today’s total is the highest case count reported on a Monday since Sept. 27, when 613 new infections were reported.

Ontario's known, active caseload is now 4,985, up from 4,040 last week.

The public health units with the highest number of cases today include Toronto (85), Peel Region (51), York Region (49), Simcoe-Muskoka (44), and Ottawa (37).

Three more virus-related deaths were confirmed today in the province.

With 21,475 tests processed over the past 24 hours, officials are reporting a provincewide positivity rate of 2.2 per cent.

Of the new cases confirmed today, 286 are in those who are unvaccinated, 23 are in people who are partially vaccinated, 212 involve those who are fully immunized and 31 have an unknown vaccination status.

Dr. Isaac Bogoch, an infectious diseases specialist at Toronto General Hospital, said while waning immunity could be partially to blame for the uptick in breakthrough infections, it is still quite a small number of cases considering about 75 per cent of Ontario’s population, including children who aren’t yet eligible to be vaccinated, are fully immunized.

Unvaccinated people make up 52 per cent of all new infections despite the fact that they only represent about 25 per cent of the population.

“Look at the proportion of people who are vaccinated versus the proportion of people who are unvaccinated and look at the true proportion of breakthrough infections. We call this the base rate fallacy as you have a massive, massive amount of the population that is vaccinated,” he told CP24 on Monday morning.

While breakthrough infections appear to be gradually rising, the number of people with COVID-19 receiving treatment in Ontario intensive care units continues to overwhelmingly be those who are not vaccinated against COVID-19.

Virus-related ICU admissions are slowly increasing in Ontario again, reaching 141 patients on Monday, up from 127 one week ago. While the province does not release data on the vaccination status of patients early in the week, as of Friday, only 14 per cent were fully vaccinated, while 86 per cent were either unvaccinated or had an unknown status.

The Saskatchewan Ministry of Health says 12 of its critically ill patients were receiving care in Ontario hospitals as of Sunday morning.

Bogoch said while vaccine protection continues to be strong, some groups could probably benefit from getting a booster shot in the next month or two.

“I think if you are looking at the sweet spot for third doses and you look at the data that is emerging from the United Kingdom, from Israel, from the United States, from a few other parts of the world, it is probably around the six-month mark,” he said.

“I think we can certainly expand who is eligible in Ontario and start lowering that number to 60 year olds and 50 year olds.”

He noted that the World Health Organization (WHO) is currently calling for a moratorium on administering booster shots to the wider community.

“I really think we will probably see some pumping of the brakes in terms of widespread, vaccine third doses for all adults until probably 2022 to adhere to WHO recommendations,” he said.

While only select groups of people in Ontario, including frontline health-care workers and people aged 70 and up, are eligible for a third jab, the province has promised to roll out booster shots to all members of the general population starting early next year.

The numbers used in this story are found in the Ontario Ministry of Health's COVID-19 Daily Epidemiologic Summary. The number of cases for any city or region may differ slightly from what is reported by the province, because local units report figures at different times.