Impact of maternal characteristics on the effect of maternal influenza vaccination on fetal outcomes

Background: Maternal infections during pregnancy have been associated with adverse fetal and infant health outcomes, and vaccination against influenza is the most effective tool to prevent morbidity and mortality due to seasonal and pandemic influenza. We evaluated the association between receipt of the inactivated seasonal influenza vaccine on preterm and small for gestational age (SGA) births, with the aim to assess racial and socioeconomic variations in vaccine effect. Methods: We conducted a retrospective analysis of state-wide surveillance data from Georgia for the most recent four years available at the beginning of the study, a total of 8393 live births in Georgia from January 1, 2005 through December 31, 2008. We constructed multivariable logistic regression models and calculated odds ratios (OR) estimates with corresponding 95% confidence intervals (CI) to evaluate the effect of maternal influenza vaccination on SGA (birth weight <10th percentile for gestational age) and preterm (gestational age at birth <37 weeks) births while controlling for potential confounders. Results: Among all women, we found significant strong associations between maternal influenza vaccination and reduced odds of a preterm birth during the widespread influenza activity period [OR = 0.39, 95% CI: 0.18, 0.83]. In this period, vaccination was protective against SGA births among women at higher risk for influenza related morbidity - women enrolled in the Women, Infant and Child (WIC) program [OR = 0.20, 95% CI: 0.04, 0.98] and Black women [OR = 0.15 95% CI: 0.02, 0.94]; maternal influenza vaccination was associated with reduced odds of a preterm birth among white women [OR = 0.34, 95% CI: 0.12, 0.91] and women of higher socio-economic status [OR = 0.30, 95% CI: 0.12, 0.74]. Conclusion: Influenza vaccination during pregnancy was significantly associated with reduced odds of small for gestational age and preterm births during the widespread influenza activity period. Vaccination effects varied by socio-demographic characteristics.