Protecting Infants with Flu Vaccines During Pregnancy

During the 2022-2023 influenza season in the US, CDC surveillance data showed that nearly half (!) of women of childbearing age (15-49yo) hospitalized with influenza were pregnant. Nevertheless, vaccination coverage in pregnant persons during the 2022-2023 influenza season was less than 50% and was approximately 5-15% lower than in influenza seasons prior to the COVID-19 pandemic. 

Infants younger than 6 months are at high risk of influenza-associated complications but are not eligible for vaccination given the absence of licensed flu vaccines for this age group. Maternal flu vaccination is safe, immunogenic, and can prevent laboratory-confirmed influenza and its complications in both mothers and infants. But recent data on the latter population in the US is limited (i.e., very little data since the H1N1 pandemic in 2009), leading to the following question: Is maternal influenza vaccination during pregnancy associated with a reduction in influenza-associated hospitalizations and ED visits in infants younger than 6 months?

In a case-control study of 3764 infants younger than 6 months at multiple sites across the US over multiple flu seasons (2016-2020), maternal flu vaccination during pregnancy was associated with a reduction in odds of medically attended influenza illness in infants younger than 6 months by one third. The magnitude of effectiveness increased with severity of infant disease, with 19% effectiveness against infant ED visits and 39% effectiveness against infant hospitalization. Vaccine effectiveness was also higher among those younger than 3 months and those born to mothers vaccinated in the third trimester.

Source: Sahni et al. JAMA Pediatr. 2024

The study’s findings also highlight important disparities. Infants born to unvaccinated mothers were more likely to have underlying conditions and to be non-Hispanic Black, publicly insured, not breastfeeding, and born prematurely. Reasons for low flu vaccine uptake are multifaceted; however, a strong recommendation from a health care professional is known to be positively associated with increased vaccine acceptability. 

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