Flu strikes fewer infants when pregnant women get vaccinated

Infants have a much lower risk of getting influenza when their mothers are vaccinated against the virus during pregnancy, a U.S. study confirms.

Doctors recommend flu vaccinations for pregnant women because the virus is linked to complications like preterm births, and because it helps protect babies from catching the flu before they're able to get vaccinated at six months of age.

Before six months, babies born to vaccinated mothers were about 64 percent less likely to have flu symptoms and 70 percent less likely to have confirmed infections than infants born to women who didn't get vaccinated during pregnancy, the study found.

"This large study provides more evidence that when women are immunized against influenza during pregnancy, their infants are much less likely to be diagnosed with influenza in their first six months of life," said lead study author Dr. Julie Shakib, of the University of Utah in Salt Lake City.

But overall, just 10 percent of mothers reported getting the flu vaccine during pregnancy, the study also found.

"Pregnant women may be concerned about harm, but they need to understand that there is strong evidence that getting vaccinated against influenza during pregnancy protects both themselves and their babies from serious harm," Shakib added by email.

To assess the prevalence and outcomes of flu vaccination during pregnancy, Shakib and colleagues analyzed data gathered from 2005 to 2014 on almost 500,000 mothers and babies.

At the time of delivery, researchers asked women if they'd been vaccinated during pregnancy. Roughly 23,000 women said they had done this, while about 222,000 said they had not.

In the first six months of life, 32 infants born to vaccinated women developed flu-like symptoms, compared with 834 babies born to unvaccinated mothers, researchers report in the journal Pediatrics.

During those first six months, 20 babies born to vaccinated women had laboratory-confirmed influenza, as did 638 infants whose mothers didn't get vaccinated, the study also found.

Researchers also looked at how waves of pandemic H1N1 strains of influenza circulating in 2009 and 2010 influenced vaccination during pregnancy.

Over four flu seasons before the pandemic, about 2.2 percent of mothers got vaccinated during pregnancy. Afterward, the average vaccination rate rose to 21 percent, with more than half of pregnant mothers getting the flu vaccine during the final flu season in the study.

One limitation of the study is its reliance on mothers to accurately recall and report whether they got vaccinated during pregnancy, the authors note. They also lacked data on when in pregnancy women received vaccines, or what strains of the virus were covered by the inoculations.

Even so, the study adds to a growing body of evidence showing how well flu vaccination during pregnancy works for protecting babies against the virus, said Dr. Matthew Kronman, an infectious disease specialist at Seattle Children's Hospital and the University of Washington who wasn't involved in the study.

"We know that pregnant women and infants are at especially high risk of severe disease with influenza," Kronman said by email. In the U.S. alone, about 100 to 150 children die of influenza each year, he added.

When pregnant women get the flu, they're at increased risk for hospitalization, intensive care and death compared to women who aren't pregnant, said Dr. Leonardo Pereira, a maternal-fetal medicine specialist at Oregon Health and Science University who wasn't involved in the study.

"Non-vaccinated women who get the flu are also contagious and can infect other people in the community," Pereira added by email.

The flu risk begins even before women leave the hospital after delivery.

"When a mother who refused the flu vaccine delivers, her newborn baby is more likely to contract influenza and more likely to require hospitalization compared to babies born to mothers who were vaccinated during pregnancy," Pereira said.