Researchers could not completely rule out an increased risk for specific defects.
The H1N1 influenza vaccine, Pandemrix, administered in pregnant women in the first 14 weeks of pregnancy does not seem to be associated with an increased risk for overall birth defects when genetic and environmental factors are considered. However, risk increases for specific birth defects could not be ruled out completely. The findings are published in Annals of Internal Medicine.
Pregnant women are a high-risk group for severe H1N1 influenza and the World Health Organization recommends vaccination for them. Research has shown conflicting risk estimates for birth defects in pregnant women receiving Pandemrix in the first trimester. Although several studies have attempted to adjust for confounding factors, none has studied siblings discordant for vaccination exposure, which, by design, would control for familial confounding (genetic and shared early environmental factors).
Researchers examined the risk for any birth defects, and specifically congenital heart disease, oral cleft, and limb deficiency, in a large population-based cohort that included more than 40,000 children of mothers exposed to Pandemrix. To minimize the influence of intrafamilial confounding, siblings were also used as comparators. The researchers found that vaccination during pregnancy did not seem to be associated with an increased risk for overall birth defects when controls from the general population were used. They also did not see an association with overall birth defects when intrafamilial confounding was taken into account. However, risk increases for specific birth defects could not be ruled out completely.