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May 31, 2011 -- Pregnant women who get a flu vaccine during flu season may be less likely to have a premature birth or deliver a baby of low birth weight.
A new study shows that women who received a flu vaccine during pregnancy and delivered during the flu season were 40% less likely to have a premature birth than were unvaccinated mothers. In addition, pregnant women who received a flu vaccine and gave birth during a period of widespread flu activity were less likely to have a baby with low birth weight.
Researchers say respiratory infections during pregnancy, such as influenza and influenza-related pneumonia, are associated with an increased risk of premature birth and low birth weight. Vaccination with a flu vaccine during pregnancy protects the mother and the infant from the most common viruses that cause influenza.
In the study, published in PLoS Medicine, researchers looked at the relationship between flu vaccination and the risk of premature birth before 37 weeks gestation and low birth weight (also known as small for gestational age) among 4,168 pregnant women who gave birth between June 2004 and September 2006.
The results showed that women who received the flu vaccine during pregnancy and gave birth during the flu season (Oct. 1-May 31) were 40% less likely to have a premature birth.
In addition, researchers found the flu vaccine's protection against premature birth increased during flu outbreaks. For example, during the eight weeks of the most widespread influenza activity, the risk of premature birth was about 70% lower among vaccinated mothers compared with mothers who did not receive the flu vaccine during pregnancy.
The study also showed that during widespread flu outbreaks, mothers who received the flu vaccine during pregnancy were 69% less likely to give birth to a low birth weight baby. Researchers did not find an association between flu vaccination and low birth weight risk during other time periods.
Researcher Saad B. Omer, of the Emory University Rollins School of Public Health in Atlanta, and colleagues say this was an observational study and can only show that there was an association between flu vaccination during pregnancy and reduced risk of premature birth and low birth weight. Further studies will be needed confirm these results and to demonstrate any causal link between the two.