A "perfect storm" of conditions during the 2014-15 flu season may have contributed to the lowest effectiveness of the annual influenza vaccine that Canadian researchers have observed in more than a decade of monitoring.

Generally, getting inoculated reduces the risk of contracting influenza by about 40 to 60 per cent. But last year's vaccine was found over all to be less than 10 per cent effective in preventing cases of flu – and in some people may have actually helped increase the risk of getting sick.

"2014-15 was a standout season for us," said Dr. Danuta Skowronski, an influenza expert at the BC Centre for Disease Control. "It was exceptional. It was the lowest vaccine effectiveness that we have recorded in the 10 years since we pioneered this approach for annual vaccine effectiveness estimations."

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That approach analyzes patient data collected from hundreds of family doctors in B.C., Alberta, Ontario and Quebec, who take part in the Canadian Sentinel Practitioner Surveillance Network. Doctors compile lab-confirmed cases of flu, as well as information on whether patients were immunized against the viruses that cause the illness.

Flu activity began earlier than usual in 2014-15 and resulted in one of the most severe influenza epidemics in recent years, with a record number of hospitalizations and outbreaks in residential care facilities, mostly affecting seniors, researchers say.

Last season's vaccine was designed to prevent infection with two A strains, H3N2 and H1N1, and a B strain, based on recommendations made by the World Health Organization near the end of the previous flu season.

But by the time the vaccine had been produced and injected into arms beginning in fall, 2014, the H3N2 virus circulating in the population had genetically mutated, a phenomenon known as drift.

Protection afforded against the mutated H3N2 strain was virtually nil, researchers found.

Not only was there a vaccine mismatch with that dominant strain – H3N2 is responsible for about 90 per cent of all flu deaths, especially among the elderly – but researchers found that people who had been inoculated in previous years actually had a higher risk of coming down with the flu than those who typically didn't get a shot.

"That's a controversial finding, obviously," said Skowronski, who led the study recently published in the journal Clinical Infectious Diseases. "But that's not a reason for us to shy away from addressing it."

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Dr. Allison McGeer, director of infection control at Mount Sinai Hospital in Toronto, said 2014-15 was the first year in "a very long time" in which a vaccine didn't work against one strain of flu virus.

"There's still a lot we need to learn about flu and a lot more that we need to understand about how flu evolves in order to get really good flu vaccines," said McGeer, who was not part of the study.

"We clearly need better vaccines and so we need people focused on how to do this better."

The atypical 2014-15 season shouldn't dissuade Canadians from getting the flu shot in the future, she added.

"If you don't get vaccinated, you don't get any protection. So you're still better off to get vaccinated," she said, noting that the 2015-16 flu shot appears to have been "pretty good" in protecting against H1N1, the most dominant influenza virus in circulation this season.

Skowronski also hopes people don't shy away from getting vaccinated every year.

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"That to me would be a tragedy because there are people – the elderly and those of any age with high-risk conditions – who in most seasons rely on the influenza vaccine to protect them from serious outcomes," she said.

"It would be sad if people turned away from the vaccine on the basis of the findings here," which Skowronski would like to see confirmed by other research groups in the United States and Europe, for instance.

"These findings should motivate investigations and improvements," she said.