Influenza vaccination administered during or after the first trimester of pregnancy has not been found to be associated with major malformations, according to the results of a study recently published in Clinical Infectious Diseases.

Although current evidence suggests that seasonal inactivated influenza vaccination during pregnancy is safe and protects both mother and newborn, mothers continue to worry about their baby’s safety. A study that examined the 2009-2010 influenza season showed no association between maternal influenza vaccination and major congenital malformations in offspring, and similar findings were reported in other small studies conducted in the United State and Canada. However, most of these studies involved a limited number of vaccines administered during the first trimester, which is when most organogenesis occurs. Since ongoing safety assessments are important to sustain vaccine uptake, a historical cohort study was conducted to analyze vaccinations stratified by trimester and ascertain major malformations through long-term follow-up of infants and the use of linked primary care, hospitalization, and mortality datasets.

In total, 78,150 live-born single infants born between 2010 and 2016 were identified from anonymized UK primary care data and included in the study. Primary care records were used to determine maternal influenza vaccination status and were stratified by trimester. Infant primary care records linked to hospitalization data and death certificates were used to obtain major malformation data. Multivariable Cox regression was used to assess the relationship between influenza vaccination and major malformations recorded in the year after delivery and in early childhood.

Results showed no evidence that the seasonal influenza vaccine was associated with major malformations when administered during or after the first trimester of pregnancy. Of the 78,150 live-birth pregnancies included, 6872 (8.8%) mothers were vaccinated in the first trimester, 11,678 (14.9%) mothers were vaccinated in the second trimester, and 12,931 (16.5%) mothers were vaccinated in the third trimester. In total, major malformations were recorded in 5707 infants in the year after delivery. After the removal of confounding factors, the adjusted hazard ratio when first-trimester vaccination was compared to no vaccination was 1.06 (99% CI, 0.94-1.19; P =.02). Similar results were observed for second- and third-trimester vaccination and for analyses that considered major congenital malformations recorded beyond the first year.

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The study authors concluded that, “There was no evidence for an association between first-trimester vaccination and major congenital malformations, limb malformations or congenital heart defects after controlling for confounding.”

Minassian C, McDonald HI, Walker JL, et al. Seasonal influenza vaccination during pregnancy and the risk of major congenital malformations in live-born infants: a 2010-2016 historical cohort study. Clin Infect Dis. doi:10.1093/cid/ciaa845/5861001