This season's flu vaccine in Canada is about 45 per cent effective, early data from doctor's offices suggest.

Several hundred volunteer physicians in British Columbia, Alberta, Manitoba, Ontario and Quebec are monitoring "in real time" how the vaccine protects people.

"We measure that the vaccine approximately cuts the risk of illness due to that virus by about half," said Dr. Danuta Skowronski of the British Columbia Centre for Disease Control, who oversees the network of doctors. "So that's substantial protection."

The estimate for effectiveness of the H3N2 strain is 45 per cent and ranges up to 50 per cent, Skowronski said.

The severity of Canada's H3N2 strain this year is similar to the intensity of the 2007/08 flu season, she added.

The sentinel system efficiently evaluates vaccine protection without the enormous costs of conducting randomized control trials and sidesteps the ethical issues of randomizing people to a vaccine or placebo, Skowronski added.

The sentinel network of doctors across Canada that evaluates vaccine protection without the enormous costs of a randomized trial, says Dr. Danuta Skowronski. (Darryl Dyck/Canadian Press)

At the beginning of this flu season, a U.S. study suggested that flu vaccine would be effective in about 59 per cent of healthy adults.

Vaccine experts say the pressure is on to invent a vaccine that offers long-lasting protection over years.

"There's lots of investigation work happening right now looking at how do we make more broadly protective influenza vaccines, something that you could give once every 10 years and it protects you against multiple strains of influenza," said Dr. Nick Kelley, who evaluates influenza vaccines at the University of Minnesota.

Skowronski said the goal is to offer complete protection, but half is still important, especially for people at high risk of serious complications.

With files from CBC's Kelly Crowe