Flu Shot for Pregnant Women Improves Fetal and Neonatal Outcomes

Pregnant women who received the H1N1 vaccine were less likely to experience stillbirth, give birth very early, or have an infant that was severely underweight, a study found.

Pregnant women who received the H1N1 influenza vaccine during the 2009-2010 pandemic were significantly less likely to experience stillbirth, give birth very early, or have an infant that was severely underweight, according to the results of a large population-based study. The study was published in the June 2012 edition of the American Journal of Public Health.

The researchers drew their data from a registry of all singleton hospital births in Ontario, Canada, between November 2, 2009, and April 30, 2010, excluding records that lacked information on whether the mother had received the H1N1 vaccine. The researchers looked at associations between the mother’s H1N1 vaccination status and risk of preterm birth (PTB), small-for-gestational-age (SGA), 5-minute Apgar score below 7, and stillbirth.

Of 55,570 mothers included in the study, 23,340 (42.0%) got the H1N1 vaccine during their pregnancy. (Based on the timing of the study, most subjects received the vaccine in the second or third trimester of pregnancy.) After adjusting for confounding factors, including mother’s smoking and socioeconomic status, the results showed that vaccinated mothers were significantly less likely to have a very PTB (less than 32 weeks’ gestation) (relative risk of 0.75), to have an SGA infant below the 10th percentile of gender-specific birth weight for gestational age distribution (relative risk of 0.90), to have an SGA infant below the 3rd percentile (relative risk of 0.81), and to have a stillbirth (relative risk of 0.66). There was practically no difference in risk for overall PTB and no association between the mother’s H1N1 vaccination status and the newborn’s 5-minute Apgar score below 7.

The researchers conclude that receiving the H1N1 vaccination in the second or third trimester was associated with significantly improved fetal and neonatal outcomes. However, they note that there was not adequate data to determine whether the benefit was directly due to the reduced likelihood of the vaccinated mothers getting the H1N1 flu.