"We're at or close to peak, certainly in southern Ontario, for influenza activity," said Dr. Doug Sider, medical director of communicable disease prevention and control at Toronto Public Health
Most of those cases come from the A/H3N2 strain of the flu.
But that strain is "not well matched" to the vaccine that was used this season, Sider said.
Officials suspect the vaccine will be less effective, though that's not yet been proven.
Health officials elsewherein Canada and in the U.S. have raised similar concerns.
Reported and lab-confirmed cases of the flu in Toronto have so far exceeded the previous ten-year average, according to Toronto Public Health. Labs confirmed another 152 new cases for the week ending Dec. 20, the most recent week for which data is available.
Authorities warned earlier this month of a "wave" of influenza cases across the city.
Hospitals and clinics are clogged, but that's not unusual for this time of year. Because family doctors often close their offices over the holidays, flu patients are more likely to go to the emergency room, Sider said.