Few debates are as divisive in Canada.
Every year, those who swear by the influenza vaccine eagerly get jabbed. Others vehemently maintain that flu shots are either useless or do more harm than good. The latter camp, however, may now have some additional ammunition.
Although doctors maintain that flu shots are life-savers that everyone should receive, some researchers are uncovering hints that "serial vaccination" -- that is, consistently receiving annual flu shots -- may in fact limit one's ability to fight the virus in the future.
"Nothing surprises me anymore with influenza," Dr. Danuta Skowronski, an epidemiologist at the British Columbia Centre for Disease Control, told CTV News. "It's such a changeable virus."
Although flu season typically starts in late November and peaks towards the end of December, influenza cases have already been reported across Canada, with spikes occurring over the past month in southern British Columbia, western Manitoba and southwestern Ontario though numbers remain low.
New evidence, however, is raising the spectre that the more influenza vaccinations you receive in your lifetime, the less protection you have from the virus in subsequent seasons.
Skowronski published a study earlier this year showing that people who were vaccinated consecutively in 2012, 2013 and 2014 appeared to have a higher risk of being infected with new strains of the flu.
"If we're seeing some signals of declining vaccine protection, we want to respond to that -- but we don't want to overreact," Skowronski cautiously says.
Skowronski's study, however, is one of several that suggest a similar troubling pattern.
In the aftermath of 2009's H1N1 flu epidemic, the Canadian flu surveillance network reported that Canadians who had received a flu shot in late 2008 were between 1.4 and 2.5 times more likely to contract an H1N1 infection that required medical attention, compared with those who didn't get a shot.
A more recent report even states that during the 2014-2015 flu season, those who had not been vaccinated the previous year were more likely to benefit from flu shots than those who received them two years in a row.
To discuss these troubling findings, Skowronski convened a meeting in Vancouver in mid-October with 40 scientists from around the world.
"The actual implications of current findings are unclear -- but important enough to warrant investigation -- to differentiate signal from noise," Dr. Mark Loeb, a scientist with McMaster University who is conducting research into the question, said in an email to CTV News.
The flu vaccine works by producing antibodies that can repel only a particular strain of influenza. Scientists alter the vaccine on an annual basis to better combat the ever-changing virus. The 2014-2015 flu season, however, was particularly bad in terms of the vaccine's usefulness. In Canada, its effectiveness was rated at a meagre nine per cent that season.
Although more research is needed, it's now believed that annual flu shots may make it harder for the immune system to fight off new strains of the virus that show up after immunization. More studies are underway in Canada, the U.S. and Europe to determine this. Quebec, however, has already put its multi-million dollar flu vaccination program under the microscope.
"The Ministry wants to know what were the findings and how can we optimize the program for the province," Dr. Gaston De Serres of Quebec's public health institute told CTV News in an email.
In the interim, doctors maintain that the flu shot is a life-saver and that everyone -- especially seniors, children under the age of two and people suffering from chronic medical conditions -- should continue receiving them.
"I think it would be premature and possibly even dangerous, frankly, for us to make strong statements in either direction, or a radical policy change, on the basis of these findings," Skowronski says.
"It would be hazardous for those individuals to stop getting the influenza vaccine on the basis of these early signals."
Such signals are so recent that public health officials don't yet know if they are real or scientific red herrings. Until more conclusive evidence is established, annual flu shot campaigns will remain unchanged.
With a report from CTV's medical specialist Avis Favaro and producer Elizabeth St. Philip