Vaccines for the 2009 swine flu virus carry a small increased risk of a rare paralyzing disease, though the benefits of the shots far outweigh their risks, according to a study funded by the U.S. government.
In a survey of about 23 million people in the U.S. who were vaccinated against the H1N1 virus responsible for the first flu pandemic in four decades, there were 77 cases of Guillain-Barre syndrome, or about 1.6 extra cases for every 1 million people vaccinated, researchers led by Daniel Salmon at the U.S. National Vaccine Program Office wrote in the online version of The Lancet medical journal.
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Previous studies on the link between flu vaccines and Guillain-Barre have had mixed results, and a survey of nine studies by the Institute of Medicine in 2011 found a causal link could not be proved or disproved. The risk identified in today's findings, based on the largest vaccination campaign in recent U.S. history, is so small it wouldn't be noticed in seasonal flu vaccine programs, Salmon and colleagues wrote.
"We cannot predict with certainty who will contract influenza, who will have a serious complication or die from the disease, or who will have a very rare but serious adverse event from the vaccine," the researchers wrote. Doctors, health officials and the general public "should be assured" that the benefits of the vaccine outweigh its risks, they wrote.
A mass flu vaccination campaign was halted in 1976 because of an association between the shot and Guillain-Barre in adults. The H1N1 vaccination campaign was the largest in recent U.S. history, and prevented as many as 1.5 million cases and 500 deaths, the researchers wrote.
In Guillain-Barre syndrome, the immune system damages nerve cells that can lead to muscle weakness and paralysis. It can occur spontaneously or after infections, according to the U.S. Centers for Disease Control and Prevention. The malady strikes between 80 and 160 Americans each year regardless of vaccination, according to the Atlanta-based CDC.