Guillain-Barre is a rare disorder, which causes a person's immune system to damage nerve cells, resulting in muscle weakness and even paralysis.
In 1976, a vaccine used during a US flu outbreak was linked to the disease, and the government halted vaccination. Debate has continued since then over whether vaccines in use could trigger the syndrome.
In a study published in the journal BMJ, a consortium of researchers reported that the H1N1 vaccine posed little, if any, increased risk for the disease.
Possible that small risk remains
"This study provides reassurance that adjuvanted pandemic influenza A (H1N1) 2009 vaccines did not increase the risk of Guillain-Barre syndrome substantially, if at all," they wrote in a journal news release.
Their conclusion stemmed from an analysis of 50 million people in five European countries that vaccinated people against H1N1 in 2009. The researchers compared 104 people with Guillain-Barre syndrome or a related condition to other people without the diseases.
The researchers adjusted their statistics to account for other risk factors for the diseases and found no link between flu vaccination and Guillain-Barre syndrome. Still, noting that it's possible a small risk might remain, they estimated there may be less than three extra cases of Guillain-Barre syndrome for each one million people who get protected by a flu vaccination.
Larger studies already in progress should provide even more definitive information, the study authors added.
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