Immunogenicity of a Monovalent 2009 Influenza A (H1N1) Vaccine Among Pregnant Women: Lowered Antibody Response by Prior Seasonal Vaccination

Background. Pregnant women are a high-risk group for influenza-associated complications and hospitalizations. Methods. To examine the immunogenicity of a monovalent 2009 influenza A (H1N1) vaccine among pregnant women, a prospective cohort study was performed at 2 medical institutes of obstetrics in Japan. One hundred fifty subjects received 2 subcutaneous doses of vaccine 3 weeks apart. The hemagglutination inhibition antibody titer was measured in serum samples collected at 3 time points: before vaccination, 3 weeks after the first dose, and 4 weeks after the second dose. Results. The first dose of vaccine induced a ≥10-fold rise in the average level of antibody. The seroresponse rate (≥4-fold rise) was 91%, and the seroprotection rate (postvaccination titer ≥1:40) was 89%. The second dose of vaccine conferred little additional induction of antibodies. Similar immune responses were observed irrespective of body mass index before pregnancy, trimester, or age at vaccination. However, lesser immune response was shown in subjects who had received the 2009–2010 seasonal influenza vaccine before the H1N1 vaccination. Conclusions. A single dose of vaccine induced an adequately protective level of immunity in pregnant women. The potential interference with seasonal vaccination requires a more thorough investigation to prepare for future influenza pandemics.