Stockholm, Sweden—Vaccination against H1N1 influenza in pregnant women doesn’t affect the risk of congenital malformation of the fetus, according to a large new Swedish study.

The article, published online by Annals of Internal Medicine, notes that earlier studies have reported varying risk estimates for congenital malformation in offspring of mothers undergoing vaccination against H1N1 influenza, known as Swine Flu, during pregnancy but did not consider the potential role of confounding by familial genetic and shared environmental factors.

A study team from the Karolinska Institutet and the Stockholm, Sweden, County Council sought to remedy that.

“It is already known that the health consequences of influenza are more severe in pregnant women than in non-pregnant women, and that influenza during pregnancy increases the risk of fetal death,” the Karolinska Institutet notes on its website about the results. “Hence, there are strong arguments to vaccinate pregnant women.”

For 2016-2017, three-component vaccines are recommended to contain A/California/7/2009 (H1N1)pdm09-like virus, A/Hong Kong/4801/2014 (H3N2)-like virus and a B/Brisbane/60/2008-like virus (B/Victoria lineage), according to the CDC. Four-component vaccines are recommended to include those viruses with the addition of B/Phuket/3073/2013-like virus (B/Yamagata lineage).

The population-based prospective study involved 40,983 liveborn offspring born between October 1, 2009, and October 1, 2011, to mothers receiving monovalent AS03–adjuvanted H1N1 influenza vaccine (Pandemrix [GlaxoSmithKline]) during pregnancy.

Of the children, 14,385 were exposed within the first trimester (14 weeks), and 7,502 were exposed during the first 8 weeks of pregnancy. Researchers compared exposed offspring with 197,588 unexposed offspring, including some siblings, to look for congenital malformation as well as for congenital heart disease, oral cleft, and limb deficiency.

Results note that congenital malformation was observed in 4.97% exposed, compared with 4.78% in nonexposed offspring, with an adjusted risk of 4.98% versus 4.96%, respectively.

Risk differences were determined to be 0.16% for vaccination during the first trimester and 0.10% for vaccination in the first 8 weeks. No statistically significant risk differences were observed in the sibling comparison, according to the report.

“When intrafamilial factors were taken into consideration, H1N1 vaccination during pregnancy did not seem to be linked to overall congenital malformation in offspring, although risk increases for specific malformations could not be ruled out completely,” study authors conclude.

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