Immunogenicity of an inactivated monovalent 2009 H1N1 influenza vaccine in pregnant women.

Background: Although pregnant women are at increased risk of severe illness following influenza infection, there is relatively little information on the immunogenicity of influenza vaccines administered during pregnancy.

Methods: We conducted a clinical trial that enrolled 120 pregnant women in which participants were randomly assigned to receive an inactivated 2009 H1N1 influenza vaccine containing either 25 μg or 49 μg of hemagglutinin (HA) in a 2-dose series with a 21-day period between administration of the first and second doses.

Results: Following the first vaccination, HA inhibition (HAI) titers of ≥1:40 were detected in 93% (95% confidence interval [CI], 82%-98%) of subjects who received the 25-μg dose and 97% (95% CI, 88%-100%) of subjects receiving the 49-μg dose.