Ontario health officials confirmed 313 new COVID-19 cases on Monday and one new death, the highest count of infections in one day since early June.

Provincial health units reported 455 new cases on June 5, with dozens of them gleaned after scrubbing data from previous days.

Ontario has reported nearly 675 cases over the past 72 hours, including 204 on Sunday.

Health Minister Christine Elliott said nearly 80 per cent of Monday's cases are from Toronto, Ottawa and Peel.

"(Seventy-eight per cent) of today's cases are from three regions, with 112 in Toronto, 71 in Peel and 60 in Ottawa. All other public health units have fewer than 10 new cases, except for York with 13 cases," she said.

Provincial labs processed nearly 30,000 test specimens over the past 24 hours.

Two thirds of the infections detected on Monday were in people under the age of 40.

“These numbers are concerning especially as we know students are now returning to school,” Ontario’s Associate Chief Medical Officer of Health Dr. Barbara Yaffe told reporters Monday afternoon.

She said provincial health officials have been in touch with the local medical officers of health for the most heavily affected regions and all of them have identified similar issues.

“As the premier and minister of health indicated earlier today the primary area of concern is social gatherings, mostly and particularly in private settings,”Yaffe said. “We do understand how hard it has been on everyone these last few months. We know there's a desire to interact with your friends and family and those outside your social circle but we are now seeing the effects of people being too casual.”

“None of us wants to return to the restrictive measures that were imposed earlier this year we've already shown that we can slow the transmission of this virus to keep our community safe and we can do it again.”

Epidemiologist Dr. Isaac Bogoch says even among young people, there is great risk posed by wider infection.

"We know people in their 20s just in general just don't get as sick as people in their 60s and 70s, but people in their 20s can still have a serious outcome from this infection, it's just not as common."

He added that the young people who are infected can very easily spread infection to older or more vulnerable populations in their homes or workplaces.

One-hundred and thirty-three people recovered from infection in the past 24 hours, leading to a net increase in active infections of 179.

There are now 2,027 active cases of infection in the province, up from less than 900 in late July.

A total of 2,816 people have died of the virus in the province since March.

Monday's test results generated a positivity rate of 1.06 per cent, the first time since June it has crossed that threshold.

Apart from Toronto York and Peel, Halton Region reported seven new cases on Monday while Durham reported nine new cases.

Ontario's centrally-reported hospitalization data continues to be incomplete.

Officials said there are now 17 patients in intensive care, with eight breathing with the help of a ventilator.

A count of data from hospitals and Ontario's 34 local public health units indicated there were 77 people in hospital for treatment of COVID-19 symptoms on Monday, far more than the 40 to 55 being reported by the province each day for the past few weeks.

Bogoch said the public has to remember that there is a time gap between infections, hospitalizations and deaths.

"There's always going to be a lag time between cases and hospitalizations and sadly another lag between hospitalizations and deaths."