A strange vaccine-related phenomenon spotted in Canada at the start of the 2009 flu pandemic may well have been real, a new study suggests.

Researchers, led by Vancouver's Dr. Danuta Skowronski, an influenza expert at the B.C. Centre for Disease Control, noticed in the early weeks of the pandemic that people who got a flu shot for the 2008-09 winter seemed to be more likely to get infected with the pandemic virus than people who hadn't received a flu shot.

Five studies done in several provinces showed the same unsettling results. But initially research outside Canada did not, and the effect was dismissed as a "Canadian problem," a problem with the flu vaccine used in Canada.

But a new study suggests the findings were real.

Skowronski and a group of researchers have recreated the event in ferrets. Their findings were presented Sun-day at the Interscience Conference on Antimicrobial Agents and Chemotherapy, a major international infectious diseases conference taking place in San Francisco.

Skowronski, who outlined the work at a webcast press conference, worked with 32 ferrets, giving half the 2008 seasonal flu shot and the rest a placebo injection. The work was blinded, meaning the researchers didn't know which ferrets received which shot. Later, all the ferrets were infected with the pandemic H1N1 virus.

The ferrets in the vaccine group became significantly sicker than the other animals, though all recovered.

"The findings are consistent with the increased risk that we saw in the human studies," Skowronski said.

The reason is unclear and Skowronski urged other research groups to take up the question. She said it's important to get to the root before the next pandemic. But in the meantime, Skow-ronski insisted the findings should not deter people from getting flu shots.