NEW YORK (Reuters Health) - Older adults who got the H1N1 flu vaccine in 2009 had a slightly increased risk of Guillain-Barre Syndrome - but the shots weren't linked to birth defects in babies of vaccinated women, according to two new studies released today.
Researchers said that even with the extra cases of Guillain-Barre, reported in eastern Canada, the flu shots still did more good than harm.
"The risk is pretty small," said Dr. Claudia Vellozzi, deputy director of the Immunization Safety Office at the U.S. Centers for Disease Control and Prevention, who wasn't involved in the new research.
On a large scale, "The influenza illness is a lot more severe, with lots of hospitalizations. I think the consensus is that the benefits certainly outweigh the risks."
Vellozzi said although U.S. studies also showed an increased risk of Guillain-Barre tied to the 2009 H1N1 vaccine, combined seasonal flu vaccines since then have protected against H1N1 but haven't been linked to extra cases of the serious muscle disorder.
Guillain-Barre is an example of an autoimmune disorder - when the immune system attacks the body, as though it's going after a foreign invader. It is most commonly seen as an after-effect of bacterial infections.
The condition is marked by nerve damage that causes muscle weakness, loss of reflexes and paralysis.
The phenomenon was a target of research during the H1N1 pandemic, in part because the "swine flu" vaccine from 1976 was linked to Guillain-Barre in the United States.
For the current study, researchers tracked newly-diagnosed cases of Guillain-Barre in the six months after H1N1 vaccination began in Quebec.
More than half of Quebec's 7.8 million residents were vaccinated, they said.
The researchers tallied 83 new cases of Guillain-Barre during the study, including 25 in people who had been vaccinated against H1N1 within the prior eight weeks.
Based on their findings, Philippe De Wals of Laval University in Quebec City and his colleagues calculated that two extra people would be expected to get the disease out of every one million vaccinated against H1N1 - a risk that was limited to people age 50 and older.
Vellozzi said those results, reported Tuesday in the Journal of the American Medical Association, were "fairly consistent" with studies done after the 2009 H1N1 vaccination campaign in the U.S.
In a related study, published in the same journal issue, researchers from Denmark found that pregnant women who were vaccinated against H1N1 - whether in their first trimester or later - were no more likely than unvaccinated women to have a baby with a birth defect or one born early.
That research involved about 53,000 women, 13 percent of whom were vaccinated in 2009 and 2010.
In a smaller sample of similar vaccinated and unvaccinated women, between four and six percent in both groups had a child with a birth defect and between seven and ten percent had a preterm birth.
There wasn't a lot of concern the vaccine might cause birth defects, Vellozzi said - but researchers always want to be particularly careful about ruling out possible risks during pregnancy.
"Our study contributes to the information available on the safety of influenza vaccination in pregnancy," study author Anders Hviid, from the Statens Serum Institut in Copenhagen, told Reuters Health in an email.
"It is by no means a complete evaluation of vaccination in pregnancy, and we cannot recommend specific courses of action based only on our study. However, our results are certainly reassuring to pregnant women and medical professionals."
Both Hviid and Vellozzi said researchers will continue to test the safety of new seasonal vaccines that are released every year, including those that contain H1N1 components.