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Did Seasonal Flu Shots
Up The Risk for H1N1 Flu?


The traditional seasonal flu vaccine may have increased the risk ofinfection with pandemic H1N1 swine flu, according to the results of four new studies by Canadian researchers.

In one study, the researchers used an ongoing sentinel monitoring system to assess the frequency of prior vaccination with the seasonal flu vaccine in people diagnosed with H1N1 swine flu in 2009 compared to people without swine flu. The researchers found that seasonal flu vaccination was associated with a 68 percent increased risk of getting swine flu.

The other three studies included additional case-control investigations in Ontario and Quebec, as well as a transmission study in 47 Quebec households that were hit with swine flu. In these studies, the researchers found that seasonal flu vaccination was associated with a 1.4- to 5.0-times greater risk of having swine flu.

The Canadian researchers add, however, that their results may signal a heretofore-undetected biological mechanism of interaction, one that could occur again in the rare instance of a pandemic strain arising after a flu season has already begun. The 2009 pandemic strain surfaced in the last weeks of the 2008-09 flu season, months after vaccines for that season had been administered.

The studies, published April 6 in the online journal PLoS Medicine, don't show whether there is a true cause-and-effect relationship between seasonal flu vaccination and subsequent swine flu illness, or whether the association was possibly due to a common factor among the people in the study, said principal investigator Danuta Skowronski, of the British Columbia Center for Disease Control in Vancouver, and colleagues.

However, the findings may raise questions about the biological interactions between pre-existing and new pandemic influenza strains.

“This illustrates the difficulty of making policy decisions in the midst of a public health crisis, when officials must rely on limited and possibly biased evidence from observational data, even in the best possible scenario of a well-established sentinel monitoring system already in place,” they write. “This perplexing experience should teach us how to best react to disparate and conflicting studies and prepare us for the next public health crisis, so that we can better manage future alerts for unexpected risk factors.”



April 8, 2010

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