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.

GENEVA, Sept 24 (Reuters) - The World Health Organisation said on Thursday it was looking into an unpublished Canadian study indicating that a seasonal flu shot could increase the risk of catching the H1N1 virus.

Marie-Paule Kieny, director of the WHO's initiative for vaccine research, said no other researchers had presented similar findings and it could be a "study bias", although the Canadian investigators were well known and capable.