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

Abstract

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. In cord blood samples, HAI titers of ≥1:40 were found in 87% (95% CI, 73%-96%) of samples from the 25-μg dose group and in 89% (95% CI, 76%-96%) from the 49-μg dose group. Microneutralization titers tended to be higher than HAI titers, but the patterns of response were similar.

Conclusions: In pregnant women, 1 dose of an inactivated 2009 H1N1 influenza vaccine containing 25 μg of HA elicited an antibody response typically associated with protection against influenza infection. Efficient transplacental transfer of antibody was also documented.

Figure 1.

Comparison of antibody titers by hemagglutination inhibition (HAI) (A) and microneutralization (B) assays in maternal delivery and infant cord blood pairs, by dose group (circles, 25 μg; crosses, 49 μg). The diagonal lines indicate equal values for the paired maternal delivery and infant cord blood specimens. Points above the line indicate a cord blood titer higher than the maternal delivery titer within each pair.

Figure 2.

Relationships between the variables of mother’s age in years at the first vaccination and interval in days between the second vaccination and delivery with the endpoints of cord blood and maternal delivery titers by hemagglutination inhibition (HAI) (A) and microneutralization (MN) (B) assays. The markers indicate the individual values for the 25-μg (circles) and 49-μg (crosses) dose groups, the lines are the estimated linear regression lines for each dose group (solid line, 25 μg; dashed line, 49 μg), and the shaded areas indicate the 95% confidence bands for each regression line.