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BOSTON – Breastfeeding boosted the protective effect of maternal flu vaccination during pregnancy for infants, a large retrospective cohort study showed.

Combined, maternal vaccination plus any exposure to human milk in the first 3 months of life was associated with a 56% lower likelihood of influenza in infants (adjusted odds ratio [aOR] 0.44, 95% CI 0.27-0.70), reported Anne-Marie Rick, MD, MPH, PhD, of the University of Pittsburgh.

Exclusive breastfeeding on top of vaccination exerted an even bigger benefit (aOR 0.37, 95% CI 0.21-0.67), according to findings detailed at the IDWeek annual meeting.

"The key message I'd like to share is that breastfeeding -- any amount of it -- really does work to help protect babies," Rick told 番茄社区app. "As clinicians, we can communicate this and try to support our moms in any way we can, because women face so many challenges to breastfeeding."

In the study, maternal vaccination alone was associated with about a 45% reduced odds of infant influenza, while breastfeeding alone was not significantly protective (aOR 0.74, 95% CI 0.49-1.12).

Children younger than 6 months are not eligible for the flu vaccine, despite having a high risk of influenza and its complications, Rick explained. These include hospitalization, need for supplemental oxygen, and secondary bacterial infections (including pneumonia and ear infections).

And while flu deaths among infants are rare in the U.S., infant influenza is a significant cause of morbidity and mortality globally, she said. "We are lucky in the U.S. that this is not something we typically see."

Extant data already suggest that breastfeeding alone can reduce the risk of respiratory infections in infants. But Rick and her colleagues wanted to specifically know how maternal vaccination and breastfeeding work together to protect infants against the flu.

To do so, they queried the University of Pittsburgh's electronic health record database from 2012 to 2019, extracting information on 44,132 mother-infant pairs who had at least two well-child visits during the infants' first year of life. The primary outcome was lab-confirmed influenza during the infant's first 6 months.

About half of the mothers (51.3%) had received the flu vaccine during pregnancy, and this was pretty evenly split across trimesters. Most of the infants of vaccinated women (60.1%) received human milk at some time during the first 3 months of life. Among the unvaccinated pairs, 57.9% received human milk at some time during the first 3 months.

A total of 141 infants (0.3%) developed laboratory-confirmed influenza between birth and 6 months of age. Models adjusted for siblings at home and low socioeconomic status.

The researchers didn't break down the protective effect by vaccine timing. But a prior study, which Rick presented earlier this year at the European Society of Clinical Microbiology and Infectious Diseases, indicated that vaccines given during the second trimester conferred the biggest protective benefit, reducing the risk of infant influenza-like illness by 79%. First-trimester vaccination reduced the risk by 39%, while vaccination during the third trimester did not protect the infant, she said.

This finding probably has to do with the time it takes to manufacture antibodies and then transfer enough of them to build up fetal immunity, she said.

"But we shouldn't be using these data to recommend timing for a maternal vaccine," said Rick. "Pregnant women are at higher risk for influenza and its complications and should get a vaccine on board as soon as possible. The vaccine is meant to protect them, so we don't want to wait to enhance any possible benefit to the baby. Any protection to the child is secondary."

However, the information can be used to help get maternal vaccination rates up, she said, especially given the hesitation among some women and even some ob-gyns about vaccination during the first trimester.

In 2017, researchers at the Marshfield Clinic Research Institute in Wisconsin, Kaiser Permanente, and the CDC, found a in a cohort of women who had the inactivated flu vaccine in 2010-2011 and 2011-2012. While the association was significant, no causative link was found. That study lingers, Rick said, giving some clinicians pause about early pregnancy maternal immunization.

"It's important to know that if you do wait to give it until the second trimester, there is this benefit to the infant," she said, "although we should never advise a woman to wait until then to get vaccinated."

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