Unpublished Canadian data are raising concerns about whether it's a good idea to get a seasonal flu shot this season.

Drawn from a series of studies from British Columbia, Quebec and Ontario, the data appear to suggest that people who got a seasonal flu shot last year are about twice as likely to catch swine flu as people who didn't.

A scientific paper has been submitted to a journal and the lead authors – Dr. Danuta Skowronski of the British Columbia Centre for Disease Control and Dr. Gaston De Serres of Laval University – won't speak to the media. Journals bar would-be authors from discussing their results publicly before they go through peer review.

While few people appear to have actually seen or read the study, the puzzling findings have been a poorly kept secret and many in the public health community in Canada have heard about them.

In fact, concern about the unconfirmed findings is playing into calls from Quebec and possibly other jurisdictions to delay or even cancel this year's seasonal flu shot campaigns across the country.

The findings are causing consternation abroad as well, with officials at public health agencies and even at the World Health Organization worried the alleged link will deter people from getting vaccinated in a fall when many people are being asked to get both seasonal and pandemic flu shots.

The Public Health Agency of Canada knows of the findings and has been seeking help here and internationally to try to figure out if the effect is real or if the studies are flawed.

"An arms-length review of the various methods is currently underway to assess the validity of the studies relative to that observation," Dr. David Butler-Jones, Canada's chief public health officer, said by email.

"We are also examining other data that will help to understand what if any association there is. We look forward to the results of the review and other data to inform our recommendations as we go forward."

The U.S. Centers for Disease Control also knows of the work. It said it has looked for similar evidence in the United States but sees none.

"It is difficult to speak about a study that has yet to be published, however, as this is an important issue involving the subject of seasonal influenza and the fast moving global pandemic of 2009 H1N1 influenza it is important to note the scientists at the Centers for Disease Control and Prevention have not seen this effect in systems we have reviewed in the United States," spokesperson Joe Quimby said by email.

A number of influenza and infectious diseases experts know of but are unwilling to speak publicly about the paper. But several were quick to note that British and Australian researchers haven't seen the phenomenon either. The lack of corroboration in other jurisdictions is "a red flag," said one expert, who does not believe the findings are true.

Another flu expert who was willing to speak on the record said they do not make sense to him either.

"I cannot think of a good reason why this is biologically likely, especially since we have sufficient evidence now that ... there is priming in the population by the way the vaccine is working," said Dr. Arnold Monto, of the University of Michigan.

He was referring to the fact that studies of swine flu vaccine show a single dose induces a strong and likely protective response in teens and adults. That suggests humankind's long exposure to seasonal H1N1 viruses has "primed" or awakened our immune systems to recognize the new virus and fight it off.

Dr. Donald Low, chief microbiologist at Toronto's Mount Sinai Hospital, was reserving judgment on the findings. But he said this kind of effect of previous exposure raising the risk of future illness is seen in some diseases, like dengue fever.

"We don't see that in flu," Monto countered.

Low said it is important to get to the bottom of the issue, but in the meantime, delaying the use of seasonal flu vaccine makes sense for logistical reasons. Swine flu activity is on the upswing in the U.S. and in parts of British Columbia, and focusing on speeding delivery of that vaccine makes more sense now, he said.

"If we're going to try to protect people, this is the virus we should be trying to protect them against," he said.

Low admitted the controversy could undermine the public's willingness to be vaccinated against influenza.

"This is obviously difficult for public to be able to digest this," he said. "There's a crying need here for a prospective randomized controlled study."

That type of study – which is not the kind on which the findings are based – is considered the gold standard of medical evidence.

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