Flu season is coming in early and strong this winter, with hospitals across Canada getting flooded with infected patients.

Most flu cases are being reported in Alberta, Ontario and Quebec, according to Canada's Public Health Agency, but there is increasing activity in Saskatchewan, Manitoba, and Newfoundland and Labrador.

"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 Public Health Ontario.

It's the same situation in Montreal, where the city's two pediatric hospitals are asking parents to keep their children at home if they have mild cold or flu symptoms. Over the past two weeks, the hospitals have received several hundred children with a cold or flu, per day.

"These are record numbers for this time of year," said Dr. Antonio D’Angelo, head of Ste-Justine's ER department.

In Manitoba, the Winnipeg Regional Health Authority (WRHA) said emergency rooms are very busy. According to WRHA, the number of people with the flu who are reporting to ERs and could not be sent home is almost double the normal rate for this time of year.

The reason for the high numbers of hospitalizations and activity could be this year's H3N2 flu strain, which is being seen in the majority of cases.

Elderly people most affected by H3N2

Public health officials said the flu vaccine this year is not as effective in protecting against the H3N2 strain, which tends to cause more deaths and hospitalizations, especially in the elderly.

According to FluWatch, Canada's national surveillance system that monitors the spread of flu, the majority of H3N2 cases have been among individuals 65 and older.

In Ontario, among the 1,169 confirmed flu cases, almost all were H3N2.

So far, the flu has caused 369 hospitalizations in the province. Nineteen people — all 65 or older — have also died, according to a public health bulletin released on Tuesday.

Despite the reduced effectiveness of the vaccine against H3N2, "the vaccine can still provide some protection against H3N2 influenza illness and can offer protection against other influenza strains such as A (H1N1) and B," according to the federal public health agency's website.

Winnipeg's health authority said that while it is hard to determine how effective this year’s vaccine has been, it is still encouraging people who haven’t yet been vaccinated to get the flu shot.

"Whether we're seeing the illness in those that were vaccinated, or whether it was people who were unvaccinated ... we won't know that probably for several weeks," said chief nursing officer Lori Lamont.