By Helen Albert
Results from two studies published in JAMA show that the H1N1 vaccination is associated with a small, but significant, increase in risk for Guillain-Barré syndrome (GBS) in adults, but does not increase the risk for birth defects or problems when given to pregnant women.
"For the vast majority of persons who received the 2009 pandemic vaccines, health benefits associated with disease prevention outweighed risks associated with vaccine administration," Philippe De Wals (University of Laval, Quebec, Canada), lead author of the GBS study, told MedWire News.
De Wals and team evaluated whether the 2009 pandemic influenza A H1N1 immunization campaign in Quebec, which resulted in 57% of the 7.8 million residents being vaccinated, led to an increase in the number of cases of the neurologic syndrome known as GBS.
The basis for this study was the unusually high number of recorded GBS cases after vaccination with inactivated "swine" influenza A vaccine in the USA in 1976 to 1977.
In total, 83 GBS cases were recorded in Quebec between October 2009 and March 2010. Of these individuals, 25 had been vaccinated against the 2009 H1N1 strain of influenza A at 8 or fewer weeks before disease onset, with most (19/25) vaccinated 4 weeks or less before GBS onset.
The researchers calculated that 2009 H1N1 vaccination increased the risk for GBS by a small, but significant amount. They estimated that the number of GBS cases attributable to vaccination was approximately 2 per 1 million doses, which is lower than the 9 cases per million doses of vaccine observed in 1976 to 1977.
Notably, there was no indication of an increased risk for GBS in people under the age of 50 years.
De Wals and colleagues are currently conducting another study to assess whether the 2009 H1N1 vaccine is associated with an increased risk for narcolepsy.
The other study, lead by Björn Pasternak (Statens Serum Institute, Copenhagen, Denmark), assessed the risks for major birth defects, preterm birth, or fetal growth restriction in children of mothers given the 2009 H1N1 vaccine during pregnancy versus children of those who were not vaccinated.
The study was based on a cohort of 53,432 Danish infants delivered between November 2009 and September 2010, of whom 6989 were exposed to the vaccine in utero.
Pasternak and colleagues found no significant differences in the incidence of major birth defects, preterm birth, or fetal growth restriction in vaccine exposed versus unexposed children regardless of trimester of exposure.
"Our study contributes to the information available on the safety of influenza vaccination in pregnancy," co-author Anders Hviid (Statens Serum Institute) told MedWire News.
"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."
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