Pregnant women who get flu shots are at no greater risk for complications like high blood pressure, urinary tract infection or gestational diabetes, according to a new U.S. study.

The results are not surprising in light of previous research, said lead author Dr. Elyse Kharbanda of Health Partners Institute for Education and Research in Minneapolis, Minnesota.

“Studies of several thousand pregnant women in scientific literature have assessed the safety of using the flu vaccine during pregnancy,” Kharbanda told Reuters Health. “These studies have shown no evidence of harm to pregnant women, to the unborn child, or to newborns of vaccinated women.”

Routine monitoring of adverse events by the U.S. Food and Drug Administration and Centers for Disease Control and Prevention also have not raised safety concerns, she noted.

Doctors already recommend that pregnant women get the shot, but not all women agree. Surveys indicate that many women are unsure of the safety of the vaccine despite evidence there are no dangers.

During the 2010-2011 U.S. flu season, for example, only about half of pregnant women reported getting a flu vaccine, Kharbanda and her colleagues point out in their report, published in the journal Obstetrics and Gynecology.

For the new study, they looked back at data on 74,000 women who had received a flu shot while pregnant, in any trimester, and compared them to 300,000 women who were similar in age, pregnancy start dates and other characteristics but had not received the shot.

Vaccinated women were more likely to have health problems, like high blood pressure or diabetes, before becoming pregnant, they found, and more likely to be hospitalized before and after the shot than the other women.

That may be explained by the fact that in general, adults with pre-existing conditions are especially encouraged to get flu shots, according to Kharbanda.

But in the 42-day period following the vaccination, women who got the shot were no more likely than women who didn’t to suffer severe morning sickness, urinary tract infections, high blood pressure, excess protein in the urine or gestational diabetes.

When they were followed through delivery, vaccinated women were still no more likely to experience those pregnancy complications or others, including preeclampsia, blood clots and other vascular problems or bacterial infection after birth.

In addition, vaccination was tied to a reduced risk of gestational diabetes, probably because women who get vaccinated are also more likely to maintain a healthy diet, the authors write.

“This is a wonderful study, I found it delightful,” said Dr. William Schaffner, chair of the department of preventive medicine at Vanderbilt University Medical School in Nashville, Tennessee.

“This is the single largest and best and most reliable safety study we have to date,” Schaffner said. “We didn’t have concerns about (the safety of the vaccine), and this puts the icing on the cake,” he said.

For a pregnant woman, contracting the flu is “really dangerous,” according to Dr. Laura E. Riley, medical director of labor and delivery at Massachusetts General Hospital in Boston.

Pregnant women with the flu are at greater risk of death, respiratory disease requiring hospitalization and premature labor and delivery, Riley told Reuters Health.

The risk-benefit ratio was already clear, she said, but collecting new safety data is always good.

“Flu shots protect pregnant women, their unborn babies, and even protect the baby after birth,” Kharbanda said.

Babies don’t receive vaccines until six months of age, so they are vulnerable to catching the flu in the first six months of life, he said. But previous studies have found that some of the protection passes across the placenta to the baby and can help shield them from flu after birth.

“What mother doesn’t want to do that?” Schaffner said. “There should be no hesitation for women getting the vaccine.”

(Reuters)