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 decisions. How can we best protect people against influenza?"
It's important to validate the information, which has not been peer reviewed, to make sure it's not just a fluke, and that the observation is confirmed elsewhere such as in the Southern Hemisphere, which just completed its seasonal flu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, or in the U.S. and UK.
Four Canadian studies involved about 2,000 people, health officials 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 or disease. 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 debating the idea of shortening, delaying or scrapping their seasonal flu vaccination campaign in favour of mass inoculation against H1N1.
The main reason is that H1N1 may be the 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 it's up to provinces and territories to decide on when to roll out flu shots.
"We don't know with this year's flu shot how it interacts with the pandemic flu shot, so it's a worry," said Dr. Michael Gardam, director of infectious diseases prevention and control at the Ontario Agency for Health Protection and Promotion.
"It makes the decision-making a lot more complex," Dr. Perry Kendall, B.C.'s provincial health officer, said in Victoria. "It would be very nice to have information cut and dried, and very clear in advance in plenty of time to make the decisions. But that isn't unfortunately the world that we're living in."
In Thornhill, Ont., Melissa Cass, who usually gets an annual flu shot to protect herself, had been planning to get one this year for her one-year-old daughter, Aliya, as well.
"You have a baby, you sort of want to protect them from everything possible. So if you can protect them from some things, why not?" Cass said.
But this year, she may not get any shots.
"If I knew that it was a risk to get H1N1, I wouldn't get any of them," she told CBC News. "I would just be as I am and just, you know, take all the precautions I possibly can for flus, and that's it."