How effective are maternal influenza vaccines during pregnancy and breastfeeding?

Conference of doctors: © Aydan-stock.adobe.com

Influenza infections disproportionately affected children aged younger than 5 years in 2022-2023, and infections tend to be most severe and result in hospitalization more often in children aged less than one year.

In her session, “Protective effects of maternal influenza vaccine during pregnancy and breastfeeding on risk of infant influenza,” Anne-Marie Rick, MD of the University of Pittsburgh in Pennsylvania presented an observational study she conducted with coauthors Helen D’Agostino, MSc; Hui Liu, MS; John V. Williams, MD; GK Balasubramani, PhD; and Judith M. Martin, MD, all from the University of Pittsburgh in Pennsylvania, on mother-infant pairs that provided insights into future strategies to help prevent infection in these most at risk patients.

A prior study demonstrated that maternal influenza vaccination during pregnancy is effective at preventing influenza infection in 56.1% of infants aged <2 months. Influenza and upper respiratory infections are also seen less often in breastfed infants. Rick and her colleagues sought to expand on these findings, and investigated whether maternal vaccination against influenza during pregnancy would provide a synergistic benefit when combined with breastfeeding to prevent influenza infection in their newborns.

The authors performed a retrospective cohort analysis of mother-infant pairs evaluating for differences in rates of influenza infections in infants. Data were obtained via electronic health record analysis. Exposures included maternal influenza vaccination during pregnancy and any human milk consumption during the first 3 months of life, and outcomes included lab confirmed influenza infection in the first 6 months of life confirmed via laboratory testing (eg, positive influenza rapid antigen or PCR testing).

In the study population, 51.3% of mothers were vaccinated against influenza, and 59.1% of the infants received human milk during the first 3 months of life.

In the study’s logistic regression model, they found an additive effect of influenza infection reductions:

• Infants whose mothers were not vaccinated against influenza, but did breastfeed had a 26% reduction in rates of influenza infection.
• Infants whose mothers were vaccinated against influenza, but did not breastfeed had a 45% reduction in rates of influenza infection.
• And infants whose mothers were both vaccinated against influenza and also breastfed had a 56% reduction in rates of influenza infection.

While exclusive human milk nutrition did enhance rates of influenza infection to 63% when combined with maternal vaccination, Rick did note that any human milk exposure was overall associated with reduced rates of influenza infection in infants.

Rick also highlighted the utility of these findings as a tool to inform clinician counseling of pregnant persons to reduce risk of influenza infection in their children. She hypothesized that exposure to human milk boosts the protection of influenza vaccination by providing broad immune bolstering effects in infants and perhaps by passing vaccine-specific antibodies directly to infants.

Future areas for investigation discussed included determining the optimal time during pregnancy and the calendar year to prevent infant infection as well as the need to study whether repeated maternal vaccination boosts the protective benefit to infants.

Reference
Rick AM, D’Agostino H, Liu H, Williams JV, Balasubramani GK, Martin JM. Protective effects of maternal influenza vaccine during pregnancy and breastfeeding on risk of infant influenza. IDWeek 2023. October 12, 2023. Boston, Massachusetts.