Canada's influenza vaccination strategies are being thrown into question, with confusion growing among Canadians after a controversial study suggesting people are twice as likely to contract pandemic H1N1 if they have received a seasonal flu shot.

The unpublished study suggests that those under 50 years of age are at a higher risk of being infected with the swine flu virus after receiving the annual flu shot.

Details of the study, coupled with skepticism in some quarters about the new H1N1 vaccine, could place territorial and provincial inoculation programs in disarray with fewer people turning up for either shot.

Lead authors, Danuta Skowronski of the British Columbia Centre for Disease Control and Gaston De Serres of Laval University, have submitted their findings to an unnamed scientific journal and may not comment until it is published.

Their paper found that consistency across four epidemiologic studies and one animal experiment suggested "an association that cannot be dismissed on the basis of chance and is unlikely to be explained entirely by bias."

British Columbia and Prince Edward Island joined Quebec, Saskatchewan, Ontario and Nova Scotia in suspending seasonal flu shots for anyone under 65