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Prenatal exposure to swine flu vaccine not associated with birth defects

Vaccination for H1N1 influenza during pregnancy is likely unrelated to congenital malformations in exposed offspring, according to study findings presented in the Annals of Internal Medicine.

“Research has shown conflicting risk estimates for congenital malformation in pregnant women receiving Pandemrix in the first trimester, with odds ratios (ORs) ranging from 0.67 to 2.18,” Jonas F. Ludvigsson, MD, PhD, of the Karolinska Institutet in Stockholm, and colleagues wrote. “Although several studies have attempted to adjust for confounding factors, none has studied siblings discordant for vaccination exposure, which, by design, would control for familial confounding (genetic and shared early environmental factors).”

Ludvigsson and colleagues performed a prospective, population-based study of offspring born between Oct. 1, 2009 and Oct. 1, 2011 to mothers who received the monovalent AS03-adjuvanted H1N1 influenza vaccine (Pandemrix) during pregnancy. They evaluated the risk for offspring malformation, taking into account familial factors.

The study included 49,983 liveborn offspring of mothers who received the vaccine prenatally (n = 40,983), within the first trimester (14 weeks; n = 14,385), and during the first 8 weeks of pregnancy (n = 7,502). Exposed offspring were compared to their unexposed siblings (n = 197,588) to control for confounding by familial factors.

Overall, 4.97% of exposed offspring (n = 2,037) vs. 4.78% of unexposed offspring (n = 9,443) had a documented congenital malformation. Adjusted analysis indicated that the risk for congenital malformation in exposed and unexposed offspring was 4.98% and 4.96%, respectively (risk difference, 0.02% [95% CI, –0.26 to 0.3]).

In addition, the corresponding risk differences for vaccination during the first trimester and within the first 8 weeks of pregnancy was 0.16% (95% CI, –0.23 to 0.56) and 0.1% (95% CI, –0.41 to 0.62), respectively. No statistically significant differences were observed between exposed and unexposed siblings.

“When intrafamilial factors were taken into consideration, Pandemrix vaccination during pregnancy did not seem to be linked to overall congenital malformation in offspring of mothers exposed to the vaccine, although risk increases for specific malformations could not be ruled out completely,” Ludvigsson and colleagues concluded. – by Alaina Tedesco

Disclosure: The researchers report funding by the Swedish Research Council and the Swedish Council for Working Life and Social Research.