Four Canadian studies involved about 2,000 people, health officials told CBC News. Researchers found people who had received the seasonal flu vaccine in the past were more likely to get sick with the H1N1 virus.
Researchers know that, theoretically, when people are exposed to bacteria or a virus, it can stimulate the immune system to create antibodies that facilitate the entry of another strain of the virus. Dengue fever is one example, Low said.
The latest finding raises questions about the order in which to get flu shots.
Across Canada, public health authorities are debating the idea of shortening, delaying or scrapping their seasonal flu vaccination campaign in favour of mass inoculation against H1N1.
