Seasonal influenza vaccination during pregnancy and the risks of preterm delivery and small for gestational age birth

Abstract

Background: Influenza vaccination is routinely recommended for pregnant women, yet information on perinatal outcomes is sparse.

Methods: We investigated the associations between trivalent (seasonal) influenza vaccination during pregnancy and the risks of preterm delivery (PTD, live birth <37 weeks gestation) and small for gestational age birth (SGA, <10th percentile in weight for sex-specific gestational age) during the influenza seasons 2006-07 through 2009-10. The study population included 1619 mothers of live-born, non-malformed singleton infants interviewed as part of the Slone Epidemiology Center's Birth Defects Study. Associations between influenza vaccination and PTD and SGA were assessed using Cox and logistic regression models, respectively, with propensity scores used to adjust for confounding. Women vaccinated against pandemic H1N1 were excluded from the analysis.

Results: Influenza vaccination during pregnancy showed a near null association with PTD for influenza seasons 2006-07 through 2008-09 compared with unvaccinated women [adjusted hazard ratios (aHR) ranged from 0.79 [95% confidence interval (CI) 0.28, 2.21] in 2007-08 to 1.08 [95% CI: 0.40, 2.95] in 2008-09]. For 2009-10, the risk of PTD was higher in vaccinated women (aHR, 7.81 [95% CI: 2.66, 23.0]). Influenza vaccination was not associated with appreciable risks for SGA for all seasons with sufficient numbers of exposed SGA.

Conclusion: Though limited by study size, these findings add support to previous observations of little or no increased risk of PTD or SGA associated with seasonal influenza vaccination for three of the four influenza seasons in our study. The increased risk of PTD observed for the 2009-10 influenza season warrants further investigation.

Keywords: infant; influenza; pregnancy; preterm birth; small for gestational age; vaccination.

Figure 1

Prevalence of seasonal influenza vaccination by study site and influenza season among mothers of non-malformed infants participating in the Slone Epidemiology Center’s Birth Defects Study, 2006–10. Seasonal coverage was estimated among 2136 women during the following influenza seasons: 2006–07 (n = 499), 2007–08 (n = 715), 2008–09 (n = 743), and 2009–10 (n = 799). Individual season numbers do not sum to total because of non-mutually exclusive influenza season exposure.