BACKGROUND:

?Some studies suggest maternal influenza vaccination can improve birth outcomes. However, there are limited data from tropical settings, particularly from Southeast Asia. We conducted an observational study in Laos to assess the effect of inactivated influenza vaccination in pregnant women on birth outcomes.

METHODS:

?We consented and enrolled a cohort of pregnant woman who delivered babies at three hospitals during Apr 2014-Feb 2015. We collected demographic and clinical information on mother and child. Influenza vaccination status was ascertained by vaccine card. Primary outcomes were the proportion of live births born small for gestational age (SGA) or preterm and mean birth weight. Multivariate models controlled for differences between vaccinated and unvaccinated women and influenza virus circulation.

RESULTS:

?We enrolled 5,103 women (2,172 [43%] were vaccinated). Among the 4,854 who had a live birth, vaccinated women were statistically significantly less likely than unvaccinated women to have an infant born preterm during the period of high influenza virus circulation (RR=0.56, 95% CI 0.45-0.70), and the effect remained after adjusting for covariates (aRR 0.69, 95% CI=0.55-0.87). There was no effect of vaccine on the proportion of infants born SGA or mean birth weight. The population prevented fraction was 18.0%.

CONCLUSIONS:

?In this observational study, we found indirect evidence of influenza vaccine safety during pregnancy, and women who received vaccine had a reduced risk of delivering a preterm live birth during times of high influenza virus circulation. Vaccination may prevent 1 in 5 preterm births that occur during periods of high influenza circulation.