Women infected with influenza virus during pregnancy are at greater risk of severe pneumonia than women of comparable age who aren’t pregnant. Further, influenza infection during pregnancy can also affect the unborn child, causing premature labor and delivery. For these reasons, in 2004 the Advisory Committee on Immunization Practices (ACIP) recommended that all pregnant women receive the inactivated influenza vaccine (IIV) during pregnancy, including during the first trimester.

Because the first trimester of pregnancy is also a time of fetal organogenesis, parents and clinicians have worried that IIV might cause harm. To address this issue, Elyse Kharbanda and colleagues, working with the Vaccine Safety Datalink, studied outcomes of pregnancies associated with first trimester receipt of IIV (Kharbanda EO, Vazquez-Benitez G, Romitti PA, et. al. First trimester influenza vaccination and risks for major structural birth defects in offspring. J Pediatr. 2017 Aug;187:234-9).

Using electronic health records from seven Vaccine Safety Datalink sites across the country, investigators compared the outcomes of 52,856 pregnancies where the mother had received IIV during the first trimester with 373,088 pregnancies where the mother had not received IIV. Selected structural abnormalities evaluated included those of the heart, face, lungs, brain, eyes, ears, intestine, muscles, limbs and genitourinary tract. The prevalence of major structural defects was 1.6 per 100 births in the vaccinated group and 1.5 per 100 births in the unvaccinated group. This difference was not statistically significant.

The authors concluded that, “First trimester maternal IIV exposure was not associated with an increased risk for selected major structural birth defects in this large cohort of singleton live births.” This study should be reassuring for pregnant mothers worried about possible harms caused to their unborn child while receiving IIV during pregnancy. 

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