Excerpt:
During the 2009 H1N1 pandemic, Canadian researchers identified a greater risk of infection in those who had been vaccinated against seasonal flu, a puzzling finding that researchers are still unraveling, including a group yesterday that revealed more about a mechanism for the process through experiments on pigs.
The heightened risk identified during the 2009 pandemic threatened to disrupt vaccination efforts in Canada, and the findings raised tough issues for policymakers, especially when studies in the United States and other countries contradicted the Canadian findings. However, in 2010 a large study by a Canadian team put an exclamation point on the earlier findings, reporting that the risk of needing treatment for pandemic flu was 1.4 to 2.4 times greater in those who had been vaccinated against seasonal flu in the previous year.
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The researchers vaccinated piglets that were younger than 6 months old and hadn't been exposed to flu before against H1N2 to explore whether vaccine-induced antibodies might play a role in exacerbating respiratory symptoms. They found some of the piglets got sick with severe pneumonia and had severe lung damage after they were infected with the 2009 H1N1 virus, suggesting that cross-reactive antibodies triggered by the flu vaccine made symptoms worse after infection with a different flu strain.
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Swine study suggests flu vaccination may sometimes backfire
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