Toronto researchers are trying to get to the bottom of the so-called “Canadian problem” — the suggestion that having a seasonal flu shot might raise one’s risk of catching H1N1 flu.

They are hoping a randomized controlled trial, the gold standard of studies, will shed some light on the puzzling observation made by other Canadian researchers earlier this year.

But the lead researcher of the new trial, which is still enrolling participants, admits they will need some good fortune to get a useful answer.

“If the pandemic vaccine works well enough, then we’re out of luck,” Dr. Allison McGeer of Mount Sinai Hospital said yesterday.

An old hand at influenza research agreed the cards may be stacked against the trial. Dr. Walter Dowdle, a retired head of virology at the U.S. Centers for Disease Control, said the unpredictable nature of influenza makes it very tough to do flu vaccine studies in humans.

The question McGeer and her colleagues are trying to answer stems from the findings of a collection of studies done across four Canadian provinces that show higher rates of H1N1 infections in people who had received a seasonal flu shot last fall.