Safety of the pandemic H1N1 influenza vaccine among pregnant U.S. military women and their newborns.

Objectives: To assess adverse pregnancy outcomes among active-duty U.S. military women who received pandemic H1N1 vaccine during pregnancy as well as adverse health outcomes among the newborns resulting from these pregnancies.

Methods: The primary study population was a retrospective cohort of active-duty U.S. military women vaccinated during pregnancy with either the pandemic H1N1 vaccine between October 2009 and June 2010 or with seasonal influenza vaccine between October 2008 and June 2009. Rates of pregnancy loss, preeclampsia or eclampsia, and preterm labor were compared between pandemic H1N1 vaccine-exposed (n=10,376) and seasonal influenza vaccine-exposed pregnancies (n=7,560). A secondary study population consisted of newborns resulting from these pregnancies. Rates of preterm birth, birth defects, fetal growth problems, and the male-to-female sex ratio were compared between newborns exposed to pandemic H1N1 vaccine and newborns exposed to seasonal influenza vaccine in utero.

Results: No significant differences were observed in rates of pregnancy loss (6.4% compared with 6.5%), preeclampsia or eclampsia (5.8% compared with 5.2%), or preterm labor (6.5% compared with 6.2%) between pandemic H1N1 vaccine-exposed and seasonal influenza vaccine-exposed pregnancies. Furthermore, no significant differences were observed in rates of preterm birth (6.2% compared with 6.3%), birth defects (2.1% compared with 2.0%), fetal growth problems (2.6% compared with 2.4%), or the male-to-female sex ratio (1.05 compared with 1.07) between newborns exposed in utero to pandemic H1N1 vaccine compared with seasonal influenza vaccine. Rates of all outcomes were lower or similar to overall general population rates. This study had at least 80% power to detect hazard ratios of 1.18-1.21 or odds ratios of 1.10-1.36, depending on outcome prevalence.

Conclusion: No adverse pregnancy or newborn health outcomes associated with pandemic H1N1 vaccination during pregnancy were noted among our cohort. These findings should be used to encourage increased vaccine coverage among pregnant women.