People who receive flu vaccines year after year can sometimes show reduced protection, an effect that Canadian infectious disease specialists say muddies public health messages for annual flu vaccine campaigns.

During the 2009 H1N1 pandemic, researchers at the B.C. Centre for Disease Control originally thought seasonal flu shots from 2008 might offer extra protection against the new pandemic strain. They were puzzled to find instead, seasonal flu vaccination almost doubled the risk of infection with pandemic flu.

Dr. Danuta Skowronski and her colleagues went on to do five more studies during the summer that showed the same effect in people and in ferrets, which are considered the best animal model of flu.

"This was a unique finding in a unique context of dramatically mismatched vaccine to novel pandemic influenza virus," she said.

What was originally called 'the Canadian problem' of higher risk of pandemic flu illness among those who received seasonal flu shots the year before has since been observed in other countries. (Spencer Platt/Getty)

She recalled the intense time, struggling to find answers about whether to offer the seasonal as well as pandemic vaccine in the fall. "That was the worst summer I think of my life, work wise."

What was originally called "the Canadian problem" has since been found in a randomized control trial by researchers in Hong Kong, with more supporting evidence from Japan and the U.S.

The paradoxical finding of increased risk of H1N1 infection only occurred during the pandemic.

Blunted protection to seasonal flu

In the 10 years since the B.C. lab invented more sensitive ways to conduct this type of sophisticated immune research, there’s never been an increase in flu illness among those vaccinated compared with the unvaccinated.

But researchers in several countries have found a blunting or "interference" effect between previous seasonal vaccines and reduced levels of vaccine protection in later years for some strains.

Regardless of previous vaccination history, flu shots did protect against flu infections requiring medical care, Huong McLean of the Marshfield Clinic Research Foundation and her colleagues concluded in a September online issue of the journal Clinical Infectious Diseases. Protection was greatest among those who weren’t vaccinated previously.

"People do not have a good explanation for why," said Dr. Michael Gardam, director of infection prevention and control at Toronto's University Health Network.

"The idea basically is that your immune system is occupied elsewhere. It would be like getting the swirling ball of death on your Mac where your operating system is doing something else rather than opening the file."

The end result of both puzzles is the same: more sickness.

Huong said it all points to the need to think about prior vaccination history when estimating vaccine effectiveness.

"These findings are so new, so emerging that policy-makers have not yet had a chance to fully digest them or understand the implications," Skowronski said.

Both Gardam and Skowronski believe the emerging findings should spur the search for a better solution.

"We have kind of hyped this vaccine so much for so long we are starting to believe our own hype. Really, what we should be doing is looking for better vaccines," Gardam said.

In the meantime, public health officials who aim to protect people from flu complications need to grapple with the imperfections of a vaccine given every year to a moving target of strains.

"I think it's reasonable that currently the recommendation is to continue, for instance, especially for people who are at high risk, immediate risk of hospitalization or death … For these people, there is a clear and present and real threat that they need to deal with and vaccine is the best way," Skowronski said.

On Friday, the Public Health Agency of Canada said regions in British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec and Newfoundland and Labrador all showed widespread flu activity last week and a large number of outbreaks in long-term care facilities.