Infants who were exposed to the flu vaccine in utero—meaning their mothers received the vaccine while pregnant—did not have an increased likelihood of adverse outcomes, according to a study today in JAMA.
The study looked at more than 99% of live births during the 2010-11 to 2013-14 flu seasons in Nova Scotia, Canada.
The World Health Organization began recommending that pregnant women be prioritized for flu vaccines in 2012 to help protect newborns as they face viruses with waning maternal antibodies and still-developing immune and pulmonary systems.
A corresponding editorial notes, however, that more pregnant women need to receive the vaccine. Authors Eduardo Azziz-Baumgartner, MD, MPH, Lisa Grohskopf, MD, MPH, and Manish Patel, MD, point out that only 61% of pregnant US women got the flu vaccine during the 2019-20 flu season, and in today's Nova Scotia study, only 36% of expectant mothers did.
The editorial authors say that, despite a wealth of data supporting flu vaccinations during pregnancy, some women are still wary about adverse outcomes to themselves or their infants because of changing or developing physiology.
"Assessing the benefits of vaccination and widely sharing the lessons from regions with maternal influenza vaccination experience or demonstration projects could have far-reaching benefits for improving the health of mothers and infants worldwide," Azziz-Baumgartner and his coauthors conclude.
The study authors also advocate for increased flu vaccine uptake in pregnant women.
No adverse outcomes linked to vaccine
The researchers used the Nova Scotia Atlee Perinatal Database and billing records for hospitalizations and physicians to follow 28,255 children an average of 3.6 years after birth (range, 2 to 5.5 years). A little over half of infants were male (51%), and about 92% were born at 37 weeks' gestation or later. About 0.4% of the province's total births were excluded from the study owing to lack of data or death.
Data showed children who were vaccinated had no statistically significant association with asthma (adjusted hazard ratio [aHR], 1.22), neoplasms (aHR, 1.26), or sensory impairment (aHR, 0.82), with crude rate differences translating to -0.17 to 0.53 increased incidences per 1,000 person-years.
Similarly, there was no statistically significant association between maternal flu vaccine and early childhood infections (adjusted incidence rate ratio [aIRR], 1.07) or increased healthcare needs (aIRR, 1.05). The researchers also found no increase in upper and lower respiratory tract infections, otitis media (ear infection), or gastrointestinal issues.
"In this population-based cohort study with mean follow-up duration of 3.6 years, maternal influenza vaccination during pregnancy was not significantly associated with an increased risk of adverse early childhood health outcomes," the researchers write. They noted that stratification by vaccination trimester and additional sensitivity analyses showed comparable results.
Infant and maternal outcomes were adjusted for variables that included rural residence, infant sex, maternal health, and sociodemographics. The flu vaccine was an adjuvanted trivalent (three-strain) inactivated flu vaccine that had two influenza A strains and one influenza B strain.
A need to address barriers, hesitancy
The researchers note that 36.2% of children were born to mothers who had received the flu vaccine during pregnancy. Pregnant women most likely to get the vaccine were over 35 years old (17.8% vs 16.6%), were first-time mothers (48.3% vs 44.4%), or had a pre-existing medical condition (such as pulmonary disease, 8.3% vs 6.5%, or asthma, 8.0% vs 6.2%).
Vaccinated women were also more likely to have a family physician at delivery (52.9% vs 40.3%) but less likely to have a history of smoking (16.2% vs 21.6%) or live in a neighborhood with the lowest mean family income (17.6% vs 21.3%).
"More research is needed to understand barriers to adoption of maternal influenza vaccination," write Azziz-Baumgartner et al in the editorial. Going beyond the study's geographic scope, they add, "Among low- and middle-income countries that have yet to adopt maternal influenza vaccination programs or that struggle to sustain influenza vaccine supplies, vaccine cost can be substantial, and information about the cost-effectiveness of vaccination is limited."
Looking toward the 2021-22 flu season, the commentators say that flu vaccines may prove to be especially important as COVID disruptions largely reduced the impact of the 2020-21 flu season.
They write, "A potential implication of markedly reduced influenza circulation and low incidence of infections is that pregnant women might not have antibodies that could protect their infants from increased risk of infection and severe illness during the next influenza epidemic. Efforts are needed now to encourage maternal influenza vaccination worldwide and to act on the compelling efficacy and safety data."