The study, published Wednesday in the Journal of the American Medical Association, suggests that use of the vaccine may have led to a doubling of the rate of cases of the rare neurological condition.
But the authors of the article said that even with an increase in Guillain-Barre cases, the benefits of the vaccination probably outweighed the risk for Quebecers.
"Obviously there is a risk. But there are benefits," said first author Philippe De Wals, a professor of community health at Laval University in Quebec City. De Wals also works for Quebec's provincial public health agency.
Guillain-Barre syndrome is a progressive but generally reversible paralysis which begins at the feet and works its way upwards. A small percentage of people who develop the condition die; most make a full recovery.
GBS, as it is sometimes called, can be caused by a number of things, including some infections. For instance, it's known that Campylobacter jejuni, a bacteria that causes gastroenteritis, can trigger Guillain-Barre syndrome.
For years there have been questions about whether getting a flu shot increases one's risk of developing the condition.
In 1976, when an outbreak of human cases of swine flu in the United States led authorities to fear a pandemic was in the offing, a mass vaccination campaign there was mounted. But the campaign was halted when a higher-than-expected number of cases of GBS were seen in people who got the vaccine, and the pandemic failed to materialize.
Since then, numerous studies have been done to try to answer the question. But the findings have been conflicting. And the same is true with studies into whether the 2009 pandemic vaccination campaigns led to an increase in Guillain-Barre cases.
In Quebec, at least among people over the age of 50, the answer appears to be yes, according to De Wals's study.
"Your baseline risk to get Guillain-Barre during a particular month is two per million. If you are vaccinated with the pandemic vaccine, your additional risk is two per million. So basically you are doubling your risk," he said.
But an influenza expert from the University of Michigan said this study, though well done, can't provide a definite answer.
"It's new wine in old bottles or the reverse," said Dr. Arnold Monto.
"It confirms what we have suspected all along, that there may be a low risk.... And the low risk, if present, is no different with adjuvanted than non-adjuvanted vaccine.
"The questions still remain."
An adjuvant is a compound that boosts the immune response triggered by a vaccine, allowing smaller doses to be used for each person.
Canada used adjuvanted flu vaccine for the first time during the 2009 pandemic. In the United States, officials opted for an unadjuvanted vaccine. But a study from the U.S. Centers for Disease Control also found a slight elevation of the rate of GBS cases after that country's pandemic vaccination campaign.
A study in Britain, where adjuvanted vaccine was used, found no increased risk.
"Sometimes you see it and sometimes you don't," said Monto. "Which to me means that there may well be a small effect but it's so small that it's hard to get statistical significance (in studies)."
In a related development, the journal also published a study looking at whether use of the pandemic vaccine in pregnant women led to increased rates of birth defects, pre-term births or other related problems.
Researchers from Denmark looked at the health records of roughly 6,500 children born to women who received H1N1 flu shots during the pandemic. They compared the children to a similarly sized group of children born during that period whose mothers hadn't taken the flu shot.
They saw no evidence of an increased risk of major birth defects, pre-term birth or fetal growth restriction.
Denmark used an adjuvanted vaccine during the pandemic, one that contained the same boosting compound — called AS03 — used in that Canada's pandemic vaccine.
Even though pregnant women are at elevated risk of becoming severely ill from flu infection — particularly during pandemics — getting them to agree to get a flu shot has been a hard sell.
In 2009, Canadian officials made a late-stage decision to make some vaccine without the adjuvant in a bid to persuade more pregnant women to get an H1N1 shot.
An editorial in the journal suggests the Danish study should assuage concerns about use of adjuvanted pandemic flu vaccine in pregnant women.
Note to readers: This is a corrected story. An earlier version misspelled Philippe De Wals's last name.