In a lengthy article published today in Public Library of Science Medicine (PLoSMed), researchers detail the results of four supplementary studies that were launched after an April 2009 school outbreak provided the first signal of an association between seasonal flu shots and pandemic flu illness. The studies, which took in about 2,700 people, found overall that the likelihood of needing medical attention for pandemic flu was 1.4 to 2.5 times greater among people who were vaccinated the previous fall.

…All four studies, the authors say, show that the seasonal flu vaccine was effective in that season, reducing cases of flu needing medical attention. But all four also show an increased risk of illness from pandemic flu among the vaccinated. The risks shown in the studies ranged from 1.19 to 3.20 times greater for those who received the seasonal vaccine than for those who did not.

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