Flu shots given to pregnant women a month or more before delivery will prevent most cases of influenza during the first six months of their babies' lives, researchers said.
"Immunize the mother and you protect the infant," Dr. Mark Steinhoff, a pediatrician with the Johns Hopkins University Bloomberg School of Public Health, said in a telephone interview.
The shots are not licensed for children younger than six months old — who are in turn more likely to be hospitalized for influenza than any other group.
In the test of 340 pregnant women in Bangladesh, the shots cut the risk of flu by 63 percent and the risk of respiratory illness overall by 29 percent. There were six confirmed cases of influenza in the vaccinated group, compared to 16 among the mothers given a different vaccine.
The injections also lowered the likelihood of fever and respiratory illness among the mothers by 36 percent.
Doctors have known for years that immunizations given to a woman can protect her newborn, so there was no reason to believe the flu vaccine would not work the same way, said Steinhoff. "We always assumed it, but nobody's done the study before," he said.
Flu shots have been recommended for pregnant women by the U.S. Centers for Disease Control and Prevention since 1997, but the advice has been widely ignored, he said. The new study, published in the New England Journal of Medicine, may change that.
"This might persuade more mothers to say, 'Hey, it really helps me and it really helps the baby,"' Steinhoff said.
Only 15 percent of pregnant U.S. women receive the vaccine each year.
The vaccinations in the Bangladesh study were given during the third trimester because in 2004 and 2005, "at the time we did the study, that was the recommendation," he said.
The current advice to pregnant women is to get the vaccine during the flu season, although it takes about a month for the protection to build in the baby.