Two separate studies published this week had somewhat contrasting findings regarding the safety of influenza A(H1N1) vaccine.
The 2009 pandemic influenza vaccine appeared to slightly increase the risk for Guillain-Barre syndrome among adults, but the vaccine was not associated with preterm birth or birth defects when given to pregnant women, according to two studies published online in the Journal of the American Medical Association.
In the first study, Philippe De Wals, MD, PhD, of Laval University, Quebec City, Canada, and colleagues assessed the risk of GBS following pandemic influenza vaccination in the fall of 2009 through March 2010.
During the 6-month period, the researchers noted 83 confirmed GBS cases. Twenty-five patients with confirmed GBS had been vaccinated against 2009 influenza A (H1N1) 8 or fewer weeks before disease onset, with most (19/25) vaccinated 4 or fewer weeks before onset. Analysis of data indicated “a small but significant risk of GBS following influenza A(H1N1) vaccination.”
The number of cases attributable to vaccination was approximately 2 per 1 million doses. The excess risk was observed only in patients aged 50 years or older, according to the study findings.
“In Quebec, the individual risk of hospitalization following a documented influenza A(H1N1) infection was 1 per 2,500 and the risk of death was 1/73,000. The H1N1 vaccine was very effective in preventing infections and complications. It is likely that the benefits of immunization outweigh the risks,” the authors wrote.
In the second study, Björn Pasternak, MD, PhD, of the Statens Serum Institut, Copenhagen, Denmark, and colleagues looked at data from a registry-based study that included all live-born singleton infants in Denmark delivered between Nov. 2, 2009, and Sept. 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. Following exclusions, a cohort of 53,432 live-born infants was identified with 6,989 (13.1%) exposed to the vaccine during pregnancy.
In a propensity score-matched 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.
In an accompanying editorial written by Mark C. Steinoff, MD, and Noni E. McDonald, MD, MSc, the authors mention how these studies mostly alleviate concern about safety of adjuvanted pandemic vaccines during pregnancy; however, they encourage more studies of pandemic influenza vaccines during pregnancy and vaccine adjuvants.
Disclosure: The first study was funded by the Ministere de la Sante et des Services sociaux du Quebec and by the Public Health Agency of Canada-Canadian Institutes for Health Research Influenza Research Network. The second study was funded by the Danish Medical Research Council. No researcher reported any conflicts of interest.
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