For pregnant women, an influenza vaccination leads to bigger babies and infants who are less likely to get the flu, according to three studies presented here.
Experts said the findings -- presented at the annual meeting of the Infectious Diseases Society of America -- might help persuade pregnant women reluctant to get a flu shot.
It might also bring the issue to the attention of obstetricians, who typically do not raise the notion of a flu shot with their patients, said Dr. William Schaffner, infectious disease specialist and chair of Preventive Medicine at Vanderbilt University Medical Center in Nashville, Tenn.
"This is powerful information for obstetricians and pregnant women to have," said Schaffner, who moderated a news conference at which the studies were discussed.
The findings are all the more persuasive, he said, because different investigators, using varied methods, "all came out with the same answer."
The issue is important, according to Dr. Marietta Vazquez, a pediatric infectious disease specialist at the Yale University School of Medicine, because the proportion of pregnant women who are vaccinated against the flu is "dismal" -- fewer than one in four, she and others said.
Vazquez and colleagues conducted a study of infants admitted to their hospital, starting in 2000. Infants with confirmed flu were the cases, and for comparison they each were matched with two babies who were admitted for other reasons.
The goal of the study is to compare the mothers -- using both questionnaires and medical records -- to see if they were vaccinated during their pregnancies, Vazquez said.
For the 119 women with complete medical records and an infant with the flu, only 5 percent had been vaccinated, the researchers found. By contrast, of the 172 mothers of control infants, 16 percent had been vaccinated.
The numbers suggested that flu vaccine given to mothers during pregnancy is effective in preventing hospitalization of their infants, Vazquez said.
Specifically, for all nonvaccinated infants, the effectiveness was 80.4 percent.
Vazquez said the results might help persuade more women to get vaccinated. "If they're not getting vaccinated for themselves," she said, "maybe they'll do it for their babies."
In a second study, researchers led by Saad Omer, assistant professor of Global health at Emory University in Atlanta, looked at prematurity and gestational weight in Georgia from June 2004 through September 2006.
During that time, Omer said, there were 6,410 births, and 15 percent of the mothers had received the flu vaccine during pregnancy.
Analysis showed that -- when flu was at least locally active -- unvaccinated mothers were 56 percent more likely to have a premature baby.
Although the findings need to be confirmed by other studies, Omer said, the flu vaccine "may be an important tool for addressing the problem of immaturity."
In a similar finding, vaccinated mothers in a study in Bangladesh had babies who were seven ounces heavier, on average, than those born to unvaccinated women, according to Dr. Mark Steinhoff, director of the Global Health Center at Cincinnati Children's Hospital Medical Center.
As in the Georgia study, the effect was seen only during the flu season, Steinhoff said.
The finding comes from a randomized trial whose primary finding, Steinhoff said, was that the flu vaccine reduced cases of influenza in babies by 63 percent.
"It shows again that when you prevent flu in a pregnant woman, you benefit the mother, you benefit the infant, and, we think, this also shows that you benefit the fetus," Steinhoff said.
Omer and colleagues reported last month, in the American Journal of Obstetrics & Gynecology, that a review of existing literature shows that pregnant women are at elevated risk of flu- related complications.
On the other hand, they said, the risk of vaccine side effects is small.
"There is substantial evidence that vaccination is not only safe for pregnant women," Omer said in a statement, "but that it is critical for protecting women and their infants against serious complications from the flu."