Infants born to mothers who received the influenza (flu) vaccine while pregnant are nearly 50 percent less likely to be hospitalized for the flu than infants born to mothers who did not receive the vaccine while pregnant, according to a new collaborative study by researchers at Wake Forest Baptist Medical Center and colleagues.

The Advisory Committee on Immunization Practices (ACIP) recommends influenza vaccination for anyone older than 6 months of age, but specifically singles out target groups, including pregnant women, who have a greater risk of influenza-related complications.

"It is recommended that all pregnant women receive the influenza vaccine during pregnancy because it is known that pregnant women have increased morbidity and mortality during pregnancy and in the immediate postpartum period if they get the flu," said Katherine A. Poehling, M.D., MPH., an associate professor of pediatrics and lead author on the study. "We also know that mothers pass antibodies through the placenta to the baby. This study showed us that receiving the influenza vaccine during pregnancy not only protects the mother, but also protects the baby in the early months of life."

The study, which is the first population-based, laboratory-confirmed study to demonstrate this benefit, appears in the June issue of the American Journal of Obstetrics & Gynecology.

Infants less than 6 months of age have the highest rates of flu hospitalization among all children, Poehling explained, yet the influenza vaccine is not licensed for or effective in infants that young.

So Poehling and colleagues sought to assess whether receiving the vaccine during pregnancy would provide some protection for the newborn baby.

The researchers analyzed data collected by the Centers for Disease Control and Prevention-funded New Vaccine Surveillance Network over the course of seven flu seasons between 2002 and 2009, before the H1N1 pandemic. The data included information about 1,510 babies who had been hospitalized with fever, respiratory symptoms, or both within the first six months of life and had undergone laboratory testing for influenza infection.

The investigators found that infants born to mothers who received the influenza vaccine during pregnancy were 45 to 48 percent less likely to be hospitalized with laboratory-confirmed influenza.

"Similar findings have been published from other studies, but they've been published in general journals or journals about pediatrics and infectious diseases," Poehling said. "Where the information is published really does make a difference because pediatricians need to know about it, but it's even more important that the doctors taking care of pregnant women – obstetricians and gynecologists (OB/GYNs) – know it, too. Pediatricians have been vaccinating children for a long time, but vaccine recommendations for OB/GYNs have changed over the last decade, so everyone is having to learn new recommendations and adjust. This is a relatively new activity for OB/GYNs."