Babies can’t get the flu vaccine until they turn 6 months old—even though children and infants die every year from the flu. But that doesn’t meant moms can’t offer their infants some protection against the flu until children are old enough to be immunized themselves, as a new study shows. In fact, babies whose mothers got the seasonal flu vaccine during pregnancy were 70% less likely to contract the flu than babies born to unimmunized mothers, thanks to two protective mechanisms working together.
“Immunizing pregnant women provides immunity to the baby through the placenta,” explained lead author Julie H. Shakib, DO, assistant professor of pediatrics at the University of Utah in Salt Lake City and medical director of the Well Baby and Intermediate Nursery. “Immunizing the mother and others who live with or care for the baby prevents them from getting the flu and passing it to the baby”—a concept known as cocooning.
It’s not clear how much each of these contributes to the reduced risk of influenza in young babies. What is clear is that vaccinating women against the flu during pregnancy, as the CDC recommends, reduces the likelihood that her baby will end up hospitalized with serious complications from influenza.
Shakib and her colleagues looked back at the records of nearly a quarter million women who gave birth in Utah or Idaho’s
Initially, just over 2% of the women reported getting the flu vaccine during pregnancy, but that number jumped tenfold to 21% after the 2009-2010 season with the pandemic strain of H1N1 influenza. By the time of the 2013-2014 flu season, 52% of pregnant women were receiving the flu vaccine, not far off from last season’s national rate of 50%.
Those variations provided the researchers with a good range for studying the rates of influenza-like illnesses and lab-confirmed flu in the women’s 249,387 children before those kids reached 6 months old. About 13 out of 10,000 babies born to moms who got the vaccine developed a flu-like illness, showing symptoms of flu but not necessarily being tested to confirm influenza. Among the children of mothers who didn’t get the flu shot, however, about 37 out of 10,000 infants developed a flu-like illness, a little over double the risk of the kids born to vaccinated moms.
The protection against actual lab-confirmed flu was even higher in the children of immunized mothers. Only 8 in 10,000 of those babies developed lab-confirmed influenza, compared to 28 out of 10,000 infants born to moms who skipped the flu shot. Only a small number of children—151 infants under 6 months old—were hospitalized for lab-confirmed flu, but the risk of hospitalization was 81% lower for babies born to immunized moms. Just 1 in 10,000 infants (3 of the 151) born to vaccinated moms ended up hospitalized while about 7 in 10,000 children born to unvaccinated moms were hospitalized.
Even though the results are associations—it’s impossible in this kind of study design to prove that it was mothers’ vaccinations that reduced their children’s risks—the results match up with previous studies that do support causation.
“Several studies have demonstrated that maternal antibodies may last for up to 6 months in infants whose mothers were immunized during pregnancy,” Dr. Shakib said. “Since children younger than 6 months are too young to be vaccinated, the best way to protect the youngest infants is to make sure their mother receives influenza vaccine during pregnancy and everyone around them is vaccinated.”
Some women worry about getting the flu vaccine during pregnancy, but a great deal of research shows both the effectiveness and the safety of doing so. Pregnant women who get vaccinated against flu are less likely to have miscarriages or stillbirths, and they’re no more likely to have children with any serious health issues or concerns than women who don’t get vaccinated.
“Obstetric providers should both recommend and have the ability to provide influenza vaccine to all pregnant women,” Dr. Shakib said. Free vaccines provided to women without insurance could further increase vaccination coverage, she added. “We hope the results of our study in a large cohort of women over a long period of time (9 influenza seasons) will help reassure expectant mothers that being immunized against influenza will help protect their infants from the serious complications of influenza,” she said.
