Public health officials are recommending that children under the age of five and those with other illnesses that can increase the risk of complications from flu infection to get vaccinated.
They say it isn't too late to get a flu shot.
Health officials in British Columbia and Alberta are seeing more flu cases. Influenza activity, including the H1N1 seasonal strain, continued to increase sharply in Canada during the last two weeks of 2013, according to the Public Health Agency of Canada's FluWatch report.
"We saw almost no influenza in British Columbia before Christmas," Dr. Paul Van Buynder, chief medical health officer for Fraser Health, said in an interview with CBC News on Monday. In the last few days, there's been a big uptick.
"We expect that there will be another four to six weeks of the severe flu season, so it's not too late to be protected. Those with chronic diseases should go out and get vaccinated."
Of the patients in intensive care with flu in B.C, among those aged 20 to 60, two were pregnant and many had other medical issues such as asthma, heart disease, diabetes or are very overweight, Van Buynder said.
- H1N1 flu surge in B.C. Lower Mainland lands people in ICUs
"So far, a greater proportion of cases nationally have been reported among people aged 20 to 64 compared with those aged 65 and older, Friday's FluWatch report said. This "is a change from the demographics of the 2012-13 season when A(H3N2) was predominant."
The majority of people who get really sick with the H3N2 strain tended to be seniors, doctors said. During last year's flu season in the U.S., more than 150 children died from flu complications, Van Buynder said, despite the myth that flu is a mild illness.
Most people who get the flu don't need medical attention, but all strains can cause serious illness.
"It's looking like a kind of a solid H1N1 season," said Dr. Allison McGeer, head of infection control at Toronto's Mount Sinai Hospital.
It's not uncommon for the flu to start on the West Coast and go across to the east, Van Buynder said.
McGeer also expects flu season will be over in about a month. "There's been a fair amount of activity, but it's not terrible. And that should be it," she said.
It can take two or three weeks for someone's immune system to build up a protective response to the flu after receiving the vaccine.