HealthDay News — Monovalent influenza A (H1N1) vaccine that contained the AS03-adjuvant was not associated with birth defects, but was linked with a slightly increased risk for Guillain-Barré syndrome (GBS), two studies show.

Infants born to pregnant women who were administered inactivated AS03-adjuvanted split virion influenza vaccine (Pandemrix) in Denmark during the H1N1 pandemic were not at increased risk for  major birth defects or smaller than average size for gestational age, Björn Pasternak, MD, PhD, from the Statens Serum Institute in Copenhagen, Denmark, and colleagues reported in the Journal of the American Medical Association.

There was no increased risk for either outcome among children exposed to the vaccine in the first, the second or third trimesters, versus unexposed infants, study data indicated.

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In a separate study published in the same journal, Philippe De Wals, PhD, from the Université Laval in Québec, and colleagues found that vaccination with a mostly inactivated monovalent AS03-adjuvanted vaccine (Arepanrix) resulted in two excess cases of GBS per million doses administered.

In an accompanying editorial, Mark Steinhoff, MD, of Cincinnati Children’s Hospital Medical Center, and Noni MacDonald, MD, of Dalhousie University in Halifax, Nova Scotia, wrote the findings “partially assuage concerns about safety of adjuvanted pandemic influenza vaccines during pregnancy.”

However, they emphasized the importance of conducting additional studies to examine the safety of other types of vaccine adjuvants.

Although previous studies examining the fetal safety of monovalent H1N1 pandemic vaccines did not reveal any cause for concern, none had directly compared outcomes among infants who were or were not exposed to vaccines during gestation.

The Pasternak study involved 53,432 liveborn singleton infants born from Nov. 2, 2009 through Sept. 30, 2010, and researchers compared birth defect, preterm birth and fetal growth restriction rates among both groups. Overall, 13.1% were exposed to the vaccine, mostly during the second or third trimester.

After propensity-score matching, major birth defects occurred in 5.5% of vaccine-exposed infants vs. 4.5% of unexposed infants (prevalence OR=1.21, 95% CI 0.60 to 2.45), the researchers found.

Preterm birth rates were 9.4% vs. 7.3% among those exposed to the vaccine in the first trimester compared with infants who were not (prevalence OR=1.32, 95% CI: 0.76-2.31). No differences were observed among the two groups in regard to low birth weight or small size for gestational age.

De Wals and colleagues studied rates of GBS among vaccinated and unvaccinated individuals, due to previous findings that vaccines administered during the 1976 swine flu pandemic increased GBS risk.

Active surveillance data collected from October 2009 through March 2010 in Quebec showed that 57% of the population of Quebec, or about 4.4 million people, received the H1N1 vaccine in the last quarter of 2002. Health officials confirmed new GBS cases in 83 individuals during the six-month study period, 25 of whom developed the disorder within eight weeks of receiving the pandemic vaccine.

In the eight-week postvaccination period, the age- and sex-adjusted relative risk was 1.80 for all confirmed cases (95% CI 1.12 to 2.87) and, in the four-week postvaccination period, the relative risk was 2.75 (95% CI 1.63 to 4.62).

Pasternak B et al. JAMA. 2012; 308: 165-174.

De Wals P et al. JAMA. 2012; 308: 175-181.

Steinhoff M, MacDonald N. JAMA. 2012; 308: 184-185.