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H1N1 flu vaccine doesn't lead to adverse pregnancy outcomes

Thursday September 26, 2013
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The H1N1 influenza vaccine appears to be safe for pregnant women, according to results of a national study.

In 2009, shortly after the H1N1 influenza outbreak, the Vaccines and Medications in Pregnancy Surveillance System launched a study to gather data on the safety of the vaccine during pregnancy. VAMPSS is a collaboration between the University of California-San Diego School of Medicine and Boson University, with coordinating assistance from the American Academy of Allergy, Asthma and Immunology.

“The overall results of this study were quite reassuring about the safety of the flu vaccine formulations that contained the pandemic H1N1 strain,” Christina Chambers, PhD, MPH, lead investigator of the UC San Diego team and director of the nonprofit Organization of Teratology Information Specialists Research Center, said in a news release.

“We believe our study’s results can help women and their doctors become better informed about the benefits and risks of flu vaccination during pregnancy.”

The team from UC San Diego followed 1,032 pregnant women across the U.S. and Canada who either chose to receive an influenza vaccine or were not vaccinated during one of the three seasons between 2009 and 2012. Women were recruited through MotherToBaby (www.mothertobaby.org), a service of OTIS.

As published this month on the website of the journal Vaccine, Chambers’ team found that women vaccinated during pregnancy were no more likely to experience miscarriage, have a baby with a birth defect or have a baby born smaller than normal compared with women who did not receive a vaccination.

Vaccinated women were more likely to have their babies before term, but only by an average of three days earlier than unvaccinated women.

The VAMPSS team from Boston University’s Slone Epidemiology Center interviewed 4,191 mothers from four regional centers in the U.S. who either had delivered a baby with one of 41 specific birth defects or delivered a normal infant. They compared the use of influenza vaccine in the two groups during the 2009-11 seasons. The team also compared the risk of preterm delivery in vaccinated versus unvaccinated women.

Overall, no significant evidence of an increased risk of any specific birth defects was noted. Although the team did observe a slight increase in preterm delivery rates among pregnant women who received the H1N1 vaccine specifically during the 2009-10 season, vaccinated women overall delivered an average of only two days earlier compared with the unvaccinated group. For those vaccinated during 2010-11, the situation was reversed, with vaccinated women less likely to deliver a preterm baby.

“We found no meaningful evidence of an increase in risk for many specific major birth defects if a woman received the flu shot early in pregnancy,” Carol Louik, ScD, lead investigator of the Boston University team, said in the news release. “A concern about the risk of specific birth defects was a critical question that has not been considered very much until now, and our data are reassuring.”

Study abstracts: www.ncbi.nlm.nih.gov/m/pubmed/24016809; www.ncbi.nlm.nih.gov/pubmed/24016804


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