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Pregnant women who received the flu vaccine during the 2009 flu pandemic lowered their risk of delivering premature babies, a new study found.

Typically flu vaccination rates among pregnant women have hovered between 13 to 18 percent nationally. But a push by health officials during the 2009 season drove vaccination rates for the H1N1 vaccine up to about 45 percent in the United States, where they have remained since.

Some expectant mothers have been reluctant to get a flu shot over concern about the health of the fetus, but the study showed that flu vaccination was not only safe but protective, said Dr. Saad Omer of the Rollins School of Public Health at Emory University, the senior author of the study.

Dr. Omer and his colleagues looked at the electronic medical records of 3,327 pregnant women between April 2009 and April 2010. The study, published in the journal Clinical Infectious Diseases, found that the infants born to vaccinated mothers had a 37 percent lower likelihood of being premature, and they also weighed more at birth than babies born to unvaccinated women.

“Our thinking is that by preventing flu infection, we are reducing the likelihood of inflammation in pregnant women and therefore having a protective effect against preterm birth,” Dr. Omer said.