Study: Exposure to H1N1 vaccine during pregnancy did not drive up rate of birth defects

A recent Danish study found that exposure to the influenza A(H1N1)pdm09 vaccine during pregnancy was not associated with a significantly increased risk of major birth defects or preterm births.

The study, published in the July 11 issue of the Journal of the American Medical Association, was led by Björn Pasternak, MD, PhD, with the Department of Epidemiology Research at the Statens Serum Institut in Copenhagen, Denmark. This is the first study to directly compare outcomes from H1N1 vaccinations during pregnancy to unvaccinated pregnancies, according to the researchers.

After examining a national registry of 53,432 infants born between November 2009 and September 2010, the researchers found that 13.1% were exposed to the H1N1 vaccine during pregnancy. A major birth defect was diagnosed in 18 of 330 (5.5%) infants exposed to the vaccine in the first trimester, compared with 15 out of 330 (4.5%) unexposed infants. Preterm births occurred in 9.4% in infants exposed to the vaccine in the first trimester, compared to 7.3% who were not exposed.

“Taken together, these studies partially assuage concerns about safety of adjuvant pandemic influenza vaccines during pregnancy,” wrote Mark C. Steinhoff, MD, and Noni E. MacDonald, MD, MSc, in a JAMA editorial accompanying the study. “However, more studies are needed examining other types of vaccine adjuvants,” Drs. Steinhoff and MacDonald added.

Dr. Pasternak and the Danish researchers also agreed that further research is needed to address the risk of specific birth defects and the effectiveness of H1N1 during pregnancy.