TORONTO -- Saving the flu shot for “when you really need it” is like playing Russian roulette, says one Ontario researcher who compared more than two dozen studies about “serial vaccinations.”

Annual flu shots remain the recommendation among doctors, despite growing evidence that repeated vaccination may weaken the body’s ability to fight the influenza viruses. That research is no reason to skip out on the shot altogether, said Jeff Kwong, a scientist with ICES and Public Health Ontario.

“Some protection is better than no protection,” said Kwong, who in January published a review of 26 studies in the journal BMC Medicine with Toronto colleagues and researchers in Hong Kong.

“Getting the influenza vaccine repeatedly, in general, is not harmful,” he told “It’s just that it may work a little bit less well.”

Kwong and fellow researchers are currently working on a new study about the impact of serial vaccinations that takes into account a 10-year history. To date, most studies have only examined effectiveness between two and five years.

Virus mutation

The “serial vaccination” question relates to a scientific concept known as the “antigenic distance” hypothesis, which theorizes that your last flu shot may interfere with the new flu shot if the vaccines are similar and the current flu strain has mutated more than anticipated.

If this year’s vaccine is similar to last year’s vaccine, the antibodies you developed from last year’s vaccine may “neutralize this year’s vaccine,” explained Kwong. And then, if the circulating viruses this year are different from last year’s vaccine, you may not get any protection from this year’s vaccine.

But what are you supposed to do with that information? That’s where a dangerous game can begin, said Kwong.

Russian roulette

You either got the shot last year and the question is whether or not to get it this year, or you didn’t get it last year and the question is still whether or not to get it this year. No one can “un-get the one from last year,” he said, so attempting to self-impose a bi-annual of tri-annual flu shot policy on yourself is a risk.

“How do you know when you’re really going to need it? It’s a little bit of Russian roulette. That’s why the recommendation is just to get it every year,” he said.

“Even if you’re getting less protection that’s better than no protection.”

Limited data

The widespread availability of flu shots everywhere from pharmacies to workplaces, and the reliability of a person’s own recollection of their history, have limited the data available to researchers. That reality means public health recommendations remain intact.

“From the policy perspective, should we tell people to get vaccinated every year or alternating years or once every three years? We don’t really know,” said Kwong.

“What we’re saying is probably it’s best just to get vaccinated every year for the most part.”