Did seasonal flu vaccines make swine flu worse?

Canadians who got a seasonal flu shot in 2008 were substantially more likely to contract the H1N1 swine flu in 2009, scientists there have found.

In four new studies, researchers compared people who got the regular flu shot the prior year with those who didn’t, and then tracked the frequency of infection with the swine-flu virus. The studies found risk increased as much as 500 percent.

The study’s authors caution that the numbers show some relationship, but can’t definitely prove prior flu vaccinations were directly responsible for the increased incidence of swine flu. But the findings do raise questions about how flu strains interact, and the World Health Organization now recommends including swine flu in new seasonal flu vaccines.