During the 2009 H1N1 pandemic, pregnant women were at increased risk of illness, death, and poor pregnancy outcomes. The researchers write:
"Pregnant women were among the main target groups prioritized for vaccination against influenza A (H1N1)pdm09, and an estimated 2.4 million women were vaccinated during pregnancy in the United States alone. However, assessment of the fetal safety of H1N1 vaccination in pregnancy has been limited to a few pharmacovigilance reports and descriptive cohort studies."
In the first study, Björn Pasternak, M.D., Ph.D., of the Statens Serum Institut, Copenhagen, Denmark and colleagues, set out to determine whether exposure to an adjuvanted influenza A(H1N1)pdm09 vaccine during pregnancy increased the risk of fetal growth restriction, preterm birth, and major birth defects.
The researchers examined 53,432 infants born in Denmark between November 2009 and September 2010. Of these infants, 6,989 (13.1%) were exposed to the vaccine during pregnancy. The team found no significant difference in the rates of major birth defects, preterm birth, or fetal growth restriction between babies whose mothers were vaccinated and those whose mothers were not.
The researchers found that:
- 5.5% of infants who were exposed to the vaccine were diagnosed with a major birth defect versus 4.5% of unexposed infants
- 9.4% of exposed infants were born preterm versus 7.3% of unexposed infants
- Preterm birth occurred in 302 of 6,543 infants (4.6%) with second- or third-trimester exposure, compared with 295 of 6,366 unexposed infants (4.6%)
"In conclusion, this nationwide cohort study in Denmark found no significant associations between exposure to an AS03-adjuvanted influenza A(H1N1)pdm09 vaccine in pregnancy and risk of adverse fetal outcomes including major birth defects, preterm birth, and growth restriction. Although the data provide robust evidence of safety with respect to outcomes associated with second- or third-trimester exposure, results from analyses of first-trimester exposure should be viewed as preliminary and need confirmation. Further research also needs to address risk of specific birth defects as well as effectiveness of H1N1 vaccination in pregnancy."
In an associated report, Mark C. Steinhoff, M.D., of the Cincinnati Children's Hospital Medical Center, and Noni E. MacDonald, M.D., M.Sc., F.R.C.P.C., of Dalhousie University, Halifax, Nova Scotia, Canada, say:
"Taken together, these studies partially assuage concerns about safety of adjuvanted pandemic influenza vaccines during pregnancy. However, more studies are needed examining other types of vaccine adjuvants. In addition, observational studies of vaccines are limited by biases, including selection bias, as well as confounding by indication. Thus, future studies with improved statistical designs including prospective follow-up studies using virological end points with adjustments for selection, seasonality, and other biases are needed to confirm these data."
Written by Grace Rattue