PHILADELPHIA — By getting a flu shot, a pregnant woman can reduce the risk that her child will be born prematurely or at a low birth weight, two studies show. Researchers working in the United States and Bangladesh presented the new data October 29 at a meeting of the Infectious Diseases Society of America.

Scientists consider flu infection during pregnancy a potential health risk to the fetus, even if the infection doesn’t make the woman outwardly sick. 

In the U.S. study, epidemiologist Saad Omer of Emory University in Atlanta and his colleagues used a database of pregnant mothers in Georgia to identify 4,168 women who gave birth between June 2004 and September 2006. Of these, 15 percent had received a flu shot during pregnancy.

Children born to women getting the shot were 40 percent less likely to be born prematurely during the entire October-to-May flu season. In the throes of flu season — the months when the most cases were reported — babies born to vaccinated moms were 70 percent less likely to be born prematurely. 

In the other study, Mark Steinhoff, a pediatrician at Cincinnati Children’s Hospital Medical Center, teamed with a Bangladeshi team in 2004 and 2005 to randomly assign 340 pregnant women to get a flu shot or a shot that protects against pneumonia and meningitis. In an earlier report, this group showed that babies born to flu-vaccinated women were less likely to get the flu in their first year of life than were newborns whose moms didn’t get that shot (SN: 11/8/08, p. 18). 

In the new analysis, the researchers found that, on average, women who didn’t get flu shots gave birth to babies weighing about a half-pound less than those born to women getting the shot. 

In the season when there is little flu in Bangladesh, the difference disappeared. “This is a fairly specific effect only seen when the virus is around,” Steinhoff says. “It’s a fairly large effect that’s statistically significant.”

In the Bangladesh study, women were immunized during the third trimester of pregnancy, which was the standard recommendation at that time. Flu shots for pregnant mothers are now approved for any trimester.

However, in the United States, flu vaccination rates among pregnant women “are dismal,” says Omer. He estimates that only 15 to 25 percent of pregnant women receive the vaccine during a given flu season. 

In part, this low rate results because obstetricians’ offices are often poorly versed on how to obtain vaccine and how to store it, says physician William Schaffner of Vanderbilt University Medical Center in Nashville, Tenn. Also, pregnant women have a long tradition of not taking anything during pregnancy, he notes. “It’s going to take a cultural change to affect that.”

Schaffner says these new data “show that fetuses will be larger and healthier at birth” if protected from flu infection through their mother. 

Although these studies didn’t include women getting the vaccine for H1N1 swine flu, that virus is particularly dangerous for pregnant women, who are considered a priority group for vaccination.