(Ivanhoe Newswire) -- In 2009 579,000 people in the United States contracted H1N1. Now, researchers are examining the risk of poor birth outcomes following H1N1 vaccination.
In studies examining the risk of adverse outcomes after receipt of the influenza A (H1N1) vaccine, infants exposed to the vaccine in utero did not have a significantly increased risk of major birth defects, preterm birth, or fetal growth restriction. In another study researchers found a small increased risk in adults of the nervous system disorder, Guillain-Barre syndrome, during the 4 to 8 weeks after vaccination.
In the first study, Björn Pasternak, M.D., Ph.D., of the Statens Serum Institute, Copenhagen, Denmark and colleagues investigated whether exposure to an adjuvanted influenza A (H1N1) pdm09 vaccine during pregnancy was associated with increased risk of major birth defects, preterm birth, and fetal growth restriction.
The registry-based study included all live-born singleton infants in Denmark delivered between November 2, 2009, and September 30, 2010. The researchers estimated the prevalence odds ratios of adverse fetal outcomes, comparing infants exposed and unexposed to an AS03-adjuvanted influenza A (H1N1) pdm09 vaccine during pregnancy. A group of 53,432 live-born infants were identified with 6,989 exposed to the vaccine during pregnancy.
In an analysis of 330 infants exposed to the vaccine in the first trimester of pregnancy and 330 unexposed, there were 18 infants diagnosed with a major birth defect among those exposed compared with 15 among the unexposed. Among infants exposed to the H1N1 vaccine in the first trimester, 31 were born preterm compared with 24 among the unexposed. Preterm birth occurred in 302 of 6,543 infants with second- or third-trimester exposure, compared with 295 of 6,366 unexposed infants.
"Taking gestational age into account, there was no increased risk of small size for gestational age associated with vaccination in the first 25exposed vs. 31 unexposed or the second or third trimester 641 exposed vs. 657 unexposed," the researchers were quoted saying.
"In conclusion, this nationwide cohort study in Denmark found no significant associations between exposure to an AS03-adjuvanted influenza A (H1N1) pdm09 vaccine in pregnancy and risk of adverse fetal outcomes including major birth defects, preterm birth, and growth restriction. Although the data provide robust evidence of safety with respect to outcomes associated with second- or third-trimester exposure, results from analyses of first-trimester exposure should be viewed as preliminary and need confirmation. Further research also needs to address risk of specific birth defects as well as effectiveness of H1N1 vaccination in pregnancy," added the researchers.
Source: JAMA, July 2012