Researchers say that flu vaccination coverage for pregnant women is still too low.
Based on their results, the study authors cite flu immunization during pregnancy as a "public health priority."
According to the Centers for Disease Control and Prevention (CDC), everyone in the United States aged 6 months or older should be vaccinated against the flu virus every year.
Flu vaccination is particularly important for children under the age of 5 years, adults aged 65 and older, and pregnant women. These groups are at the greatest risk for flu-related complications.
Now, a new study further highlights the importance of flu vaccination for expectant mothers. The study found that babies of mothers who receive the shot during pregnancy are much less likely to develop flu in the first 6 months of life.
"Babies cannot be immunized during their first 6 months, so they must rely on others for protection from the flu during that time," notes lead author Julie H. Shakib, assistant professor of pediatrics at the University of Utah School of Medicine.
"When pregnant women get the flu vaccine there are clear benefits for their infants."
97 percent of flu-infected infants born to non-vaccinated mothers
To reach their findings, Shakib and colleagues analyzed the health records of more than 245,000 pregnant women and their offspring - which totaled more than 249,000 infants, including twins and triplets, and even larger multiple births. The researchers looked at health records over nine flu seasons between December 2005 and March 2014.
Around 10 percent of the women reported getting the flu shot during pregnancy, while the remaining 90 percent were not vaccinated.
Over the nine flu seasons, laboratory-confirmed flu was confirmed among 658 infants aged 6 months or younger. Of these cases, 638 (97 percent) occurred among infants whose mothers had not received the flu vaccination during pregnancy.
A total of 151 of the infants with laboratory-confirmed flu were hospitalized, the researchers report, and 148 of these infants were born to mothers who had not been vaccinated against flu during pregnancy.
The researchers calculated that flu vaccination during pregnancy reduced the risk of laboratory-confirmed flu by 70 percent and the risk of flu-related hospitalization by 80 percent for infants aged 6 months and younger.
On analyzing the health records for incidence of respiratory syncytial virus (RSV) - a respiratory infection that is common during the winter months - the team found that flu shot during pregnancy had no impact on RSV incidence among infants.
The researchers say this finding suggests that the lower risk of flu among infants whose mothers were vaccinated during pregnancy is a result of the vaccine itself.
Flu vaccination coverage remains too low for expectant mothers
Senior study author Dr. Carrie L. Byington, professor of pediatrics at the University of Utah School of Medicine, notes that flu vaccination rates among expectant mothers increased during the 2009-2010 swine flu (H1N1) pandemic.
However, she says that such rates are still too low, representing a "public health issue."
"About 50 percent of pregnant women reported being immunized in the latest flu season," says Dr. Byington. "But we need to get that number much closer to 100 percent."
The researchers stress that both pregnant women and their offspring are at increased risk for flu-related complications, highlighting the importance of immunization.
"Pregnant women are a high-risk group during influenza season and influenza outbreaks and should receive vaccinations. If their caregivers don't offer them influenza vaccinations, I would encourage all pregnant women to ask them for it."
Study co-author Dr. Michael W. Varner, University of Utah School of Medicine
Shakib says she and her team hope their findings encourage more women to get the flu shot.