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New CDC Observational Study: The Effect of Influenza Vaccination on Birth Outcomes

A new observational study from the CDC, published this week in the journal Clinical Infectious Diseases, found that women who received an influenza vaccine during pregnancy had an approximate 20% reduced risk of delivering a preterm baby during times of high flu activity, preventing up to 1 in 5 preterm births. The study, which was conducted among pregnant women in Lao People’s Democratic Republic (Lao PDR), also found that for every week of protection against flu after being vaccinated, women had an additional 4% reduced risk of preterm birth.

Flu infection in pregnant women has long been associated with higher risks of severe birth outcomes both during the flu season and during flu pandemics, as well as an increased risk of preterm birth and neonatal death. Studies have shown that when pregnant women receive a seasonal flu vaccine, they not only have reduced incidence of flu, but their infants have a reduced risk of flu during their first 6 months of life. This study set out to learn if flu vaccination among pregnant women could also have an indirect effect on birth outcomes. Given the global mortality burden and cost associated with infants who have poor birth outcomes, there are important implications of a protective vaccine effect. While national vaccination programs have increased in some parts of the world, few countries in Southeast Asia recommend influenza vaccine in pregnant women.

Between April 2014 and February 2015, researchers enrolled 5,103 women at three hospitals in Laos at the time of either delivery or miscarriage and collected clinical and demographic data on mother and child. Of these women, 43% were vaccinated against flu. Medical records and vaccine registries are not regularly maintained at health care facilities in Laos; however, pregnant women are provided a booklet to record prenatal visit information and are advised to bring the booklets to all prenatal visits and delivery. Additionally, vaccination cards were provided to all pregnant women who participated in a seasonal flu vaccination campaign between March 20 and June 30, 2014. Researchers considered women as “vaccinated” who provided documentation of vaccination at least 14 days prior to delivery.

Researchers measured three birth outcomes for the vaccinated versus unvaccinated group of women: small for gestational age (SGA), mean birth weight and preterm (defined as born prior to 37 weeks). Among the live births (4,854), vaccination showed no effect on SGA or mean birth weight; however, results showed that vaccinated women were less likely than unvaccinated women to have a preterm birth during periods of high flu activity. Since flu infection leads to inflammation, a known cause for preterm birth, it is logical that flu vaccine could help prevent preterm birth.

These findings provide additional indirect evidence of the safety of influenza vaccines during pregnancy (as there was no heightened risk of adverse birth outcomes to vaccines) and provide evidence that flu vaccine may have a protective effect on preterm birth. CDC and the Advisory Committee on Immunization Practices recommend that pregnant women get a yearly flu shot.

Read the full study report in the journal Clinical Infectious Diseases.

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