A pregnant woman who gets a flu shot passes protection on to her fetus that lessens the newborn’s likelihood of contracting the flu during the first months of life, researchers report in the Oct. 9 New England Journal of Medicine.

Although the vaccine has been shown to be safe, no randomized trial has evaluated the shot’s effectiveness in a clinical setting — until now.

“I think this will now make a difference,” says study coauthor Mark Steinhoff, a pediatrician at Johns Hopkins University in Baltimore and the Cincinnati Children’s Hospital Medical Center. “If you want to protect the baby and be careful, maybe the vaccine is a way to do that. I think more women will ask for it,” he says.

Vaccinating pregnant women against influenza is approved and even recommended by U.S. medical authorities and by the World Health Organization, but few mothers-to-be get a shot.

“Pregnant women are generally healthy, and healthy people often don’t seek out preventive measures like vaccinations,” says Lisa Jackson, an internist and vaccine researcher at the Group Health Center for Health Studies in Seattle. “And a lot of pregnant women want to avoid extraneous medicines and other exposures.” What’s more, she says, obstetricians aren’t focused on vaccinations during a woman’s pregnancy and may not have flu-shot kits on hand.

Flu is dangerous to newborns. In some years, fully 1 percent of all newborns under age 6 months in the United States are hospitalized for flu during the peak winter season, a previous study found. Infants don’t receive flu shots directly until they reach 6 months of age

In the new study, Steinhoff and his team recruited 340 pregnant women in Bangladesh and randomly assigned about half to get the standard flu vaccine by injection. The other women served as a control group, receiving a vaccination that protects against pneumonia and meningitis. That shot is beneficial but has no impact on flu susceptibility. Each woman received one shot in the third trimester. The nature of the shot was concealed until after the trial.

After the women gave birth, weekly visits to clinics revealed that six infants whose mothers had received flu shots developed a diagnosed case of the flu during their first six months, whereas 16 babies born to the other mothers did.

The babies whose mothers got flu shots also were roughly one-third less likely to contract a nonspecific respiratory infection accompanied by a fever — illnesses that were probably undiagnosed influenza, Steinhoff says.

When vaccinated, a pregnant woman makes antibodies against the flu virus and passes some of them along to her fetus. The new data suggest that these antibodies provide a grace period for a newborn until they wear off months later, Steinhoff says.

“This certainly does suggest there’s a benefit to flu shots during pregnancy,” says Jackson. The magnitude of the benefit might be greater in Bangladesh — where flu is a year-round problem — than in the West, where it’s seasonal, she says. “But this shows a ‘proof of concept’ for sure — that you can reduce the risk of flu in an unvaccinated [newborn] population by a substantial amount.”

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