Study: Infants of moms who had flu shot in pregnancy at 39% lower risk of hospitalization – University of Minnesota Twin Cities

The infants of mothers vaccinated against influenza during pregnancy had a 39% lower risk of flu-related hospitalization than those born to unvaccinated mothers, estimates a study published today in JAMA Pediatrics.

For the test-negative case-control study, the New Vaccine Surveillance Network Collaborators examined the association between maternal flu vaccination and severe disease among 3,764 infants younger than 6 months at seven pediatric care centers in seven states during the 2016-17 through 2019-20 flu seasons.

The researchers noted that flu during pregnancy is tied to severe maternal disease and may be linked to poor birth outcomes such as preterm birth, fetuses small for their gestational age, and miscarriage. Infants aren't eligible for flu vaccination until they are 6 months old.

"Maternal immune responses to influenza vaccine during pregnancy are comparable to those of nonpregnant adults, and the transfer of influenza antibodies, either naturally acquired or vaccine induced, from mother to fetus is highly efficient," they wrote.

Among the 3,764 infants, 53% were born to mothers who received the flu vaccine during pregnancy, including 42% of 223 flu-positive infants and 54% of 3,541 flu-negative controls.

Overall, vaccine effectiveness (VE) of maternal vaccination against flu-related ED visits or hospitalizations in infants was 34%. Maternal flu vaccination was 19% effective against infant ED visits, 39% against infant hospitalization, 25% against influenza A, and 47% against influenza B in infants. By subtype, VE was 39% and 16% against the H1N1 and H3N2 strains, respectively.

VE was 53% among infants younger than 3 months, 52% among those whose mothers were vaccinated in the third trimester, and 17% in those whose mothers were vaccinated in the first or second trimester.

Among 223 infants with flu, 28% had retractions (pulling in between the ribs), and 10% had wheezing in the first 24 hours of life. More than half (56%) of flu-positive infants were hospitalized, 5% required intensive care unit admission, 14% required supplemental oxygen, and 1% required intubation.

"The findings in this study indicate that maternal influenza vaccination during pregnancy provided important protection for the infant in the first few months of life before infants are eligible for vaccination," the study authors wrote.

The World Health Organization and numerous other agencies and associations recommend that pregnant women receive the inactivated flu vaccine.

While continued efforts are needed to determine optimal timing, clinicians should continue to offer influenza vaccination at any time during pregnancy to protect both the pregnant person and infant.

"Despite this recommendation, global maternal influenza vaccine uptake remains suboptimal," the investigators wrote. "In the US, vaccination coverage in pregnant persons during the 2022 to 2023 influenza season was less than 50% and was approximately 5% to 15% lower than in influenza seasons prior to the COVID-19 pandemic."

"While continued efforts are needed to determine optimal timing, clinicians should continue to offer influenza vaccination at any time during pregnancy to protect both the pregnant person and infant," they added.

In a related commentary, Matthew Zuber, MD; April Miller, MD, MPH; and Katherine Poehling, MD, MPH, all of the Wake Forest School of Medicine, said that vaccine registries must expand from children to people of all ages to eliminate gaps in record-keeping and that obstetric and pediatric clinicians need to share effective approaches in their communities to enhance access, confidence, and coverage of vaccines and preventive care.

"Our patients need us to advocate for universal vaccine registries and roll up our sleeves and work collaboratively to optimize maternal and infant health for all," they wrote

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Study: Infants of moms who had flu shot in pregnancy at 39% lower risk of hospitalization - University of Minnesota Twin Cities