Preliminary research suggests the seasonal flu shot may put people at greater risk for getting swine flu, CBC News has learned.
"This is some evidence that has been floated; it hasn't been validated yet, it's very preliminary," cautioned Dr. Don Low, microbiologist in chief at Mount Sinai Hospital in Toronto.
"This is obviously important data to help guide policy decision. How can we best protect people against influenza?"
It's important to validate the information to make sure it's not just a fluke, and that the observation is confirmed elsewhere such as in the Southern Hemisphere, which just completed its seasonal flu season, Low added.
The four Canadian studies involved about 2,000 Canadians in Ontario, Quebec, British Columbia and Alberta, sources told CBC News. Researchers found people who had received the seasonal flu vaccine in the past were more likely to get sick with the H1N1 virus.
Researchers know that theoretically when people are exposed to bacteria or a virus, it can stimulate the immune system to create antibodies that facilitate the entry of another strain of the virus. Dengue fever is one example, Low said.
No seasonal flu shot?
The latest finding raises questions about the order in which to get flu shots.
Across Canada, public health authorities are fiercely debating the idea of shortening, delaying or scrapping their seasonal flu vaccination campaign in favour of mass inoculation against H1N1.
The main reason is because the H1N1 may be dominant strain of influenza circulating when the fall flu season hits, meaning it could be a waste of time and resources to mount a seasonal flu vaccine campaign.
Health authorities in Quebec are considering cancelling or postponing seasonal flu shots for some groups, such as healthy, younger adults.
The Public Health Agency of Canada says its up to provinces and territories to decide on when to roll out flu shots.
"You can imagine the people over the age 65 who are relatively protected against H1N1, it may make perfect sense for them to get the seasonal vaccine. People who are younger than that, if we don't see a lot of seasonal flu, probably the pandemic vaccine's the way to go. But we're going to make that crystal clear to people," said Dr. Michael Gardam, director of infectious diseases prevention and control at the Ontario Agency for Health Protection and Promotion.
Seasonal flu could also appear after Christmas, said Dr. David Scheifele, director of the vaccine evaluation centre at B.C. Children's Hospital in Vancouver.
With files from The Canadian Press