First-trimester maternal inactivated influenza vaccine (IIV) was not linked to an increased risk of major structural birth defects in singleton infants, said Elyse Olshen Kharbanda, MD, of HealthPartners Institute, Minneapolis, and her associates.

Data from seven participating Vaccine Safety Datalink sites in six states were used to identify 52,856 women who received IIV in the first trimester of pregnancy (12% of the study total) and 373,088 not exposed to the flu vaccine in the first trimester (88%). A total of 865 women in the IIV-exposed group had an infant with 1 of the 50 selected major structural defects (1.6 per 100 live births), versus 5,730 in the unexposed group (1.5 per 100 live births).

The adjusted prevalence ratio for having one of the birth defects after being exposed to IIV in the first trimester was 1.02 (95% confidence interval, 0.94-1.10). There were no increased risks for any of the major structural birth defects after maternal first-trimester IIV, including cardiac defects, neural tube defects, microcephaly, or cleft lip and/or cleft palate.

Among the strengths of the study were the large population, which allowed the researchers to examine subgroups of major structural birth defects; their findings were consistent across all those subgroups. In addition, the investigators were able to exclude women at increased risk for major birth defects because of comorbidities such as diabetes, drug exposures, diagnosed chromosomal abnormalities, or congenital infections. Finally, the study authors were able to exclude women with potential exposure to teratogenic medications.

“Because IIV is currently recommended for all women who will be pregnant during periods of influenza circulation, these data should provide reassurance for women considering first trimester vaccination,” said Dr. Kharbanda and her associates.

Read more in the Journal of Pediatrics (2017 May 24. doi: 10.1016/j.jpeds.2017.04.039 ).