A low vaccination rate among pregnant women to prevent whooping cough in their babies might be explainable, considering that the federal recommendation to receive the vaccine during each pregnancy is only four years old.

But a state survey of flu shots among pregnant women found the rate to be worse. And they have been recommended for decades.

Together, the results disappointed public health advocates who want to use maternal vaccinations to prevent disabling and deadly diseases from spreading to newborns.

Reviewing records for 113,730 women who gave birth in 2013 and 2014, officials from the Minnesota Department of Health found that 58 percent received the Tdap shot for tentanus, diphtheria and pertussis (whooping cough). Only 46 percent received flu shots during their pregnancies, despite evidence that both vaccines pass from mothers to their fetuses and protect them at birth.

"This is really to protect those most vulnerable before they are old enough to receive their own vaccinations," said Anna Fedorowicz, an immunization coordinator for the state Health Department. She co-authored a report on the survey results that was recently published by the U.S. Centers for Disease Control and Prevention.

The results mirrored national data, but the state survey was unique in that it looked at racial, ethnic and socioeconomic differences in Minnesota. Vaccinations were less likely among American Indians and women born in Somalia or Eastern Europe.

Doctors have found many Somali women reluctant to vaccinate their children against measles, mumps and rubella because of fears that the MMR shots have a link to autism.

"Is that isolated, or is it potentially cultivating the sense of fear or concern about … other vaccinations too?" Fedorowicz asked.

Vaccinations were less likely among women who were uninsured, received inadequate prenatal care and had less formal education.

A doctor's recommendation can make a big difference, officials say, so the Health Department has privately analyzed the data by individual clinicians and contacted doctors with the lowest rates.

"Having that data to point back to has been informative for providers," said Miriam Muscoplat, a co-author of the report, "who generally think they've been doing really, really awesome at things when the data doesn't necessarily show that."