Antibody response to Influenza A(H1N1)pdm09 in vaccinated, serologically infected and unaffected pregnant women and their newborns

OBJECTIVE: To evaluate the serological response in pregnant Danish women immunized during the 2009 pandemic by serologic infection or by vaccination with influenza A (H1N1) Pandemrix(®) and describe levels of passively acquired maternal antibody in their offspring.

DESIGN: Observational cohort study.

SETTING: Department of Obstetrics, Aarhus University Hospital, Skejby, Denmark, October to December 2009.

POPULATION: Pregnant women and their offspring.

METHODS: Serological analysis of antibodies to Influenza A (H1N1)pdm09 by hemagglutination inhibition assay in 197 women and their offspring. Blood samples were collected consecutively at delivery from the mother and the umbilical cord. In a subgroup of 124 of the 197 women, an additional blood sample from gestational week 9-12 was available for analysis.

MAIN OUTCOME MEASURES: Seroconversion, geometric mean titer, geometric mean-fold rise and protective antibodies.

RESULTS: Thirty-three of the 124 subgroup women (27%) seroconverted during pregnancy. Seventy-nine percent after vaccination, and 17% after serologic infection (p<0.001). The geometric mean titer after delivery in non-vaccinated, non-serologically infected women was 17.1 [95%CI 15.7-18.6]. The geometric mean titer increased significantly after serologic infection with H1N1 (76.5 [95%CI 51.3-113.9], p<0.001) and after vaccination (589.6 [95%CI 339.3-1024.7], p<0.001). The geometric mean-fold rise (mother at delivery/mother early pregnancy) was significantly higher after vaccination (2.23 [1.93-2.54]) than serologic infection (1.73 [1.59-1.87], p=0.013). In newborns of vaccinated mothers 89.5% had protective antibody levels compared to 15.8% in newborns of serologically infected mothers (p<0.001).

CONCLUSIONS: Influenza vaccination during pregnancy confers passive immunity to the newborn. This article is protected by copyright. All rights reserved.