Effectiveness of an influenza A (H1N1) 2009 monovalent vaccine among Japanese pregnant women: A prospective observational study assessing antibody efficacy

► Effectiveness of an influenza A (H1N1) 2009 monovalent vaccine was estimated. ► We prospectively observed 135 Japanese pregnant women. ► Vaccine effectiveness was calculated using an index of “antibody efficacy”. ► Antibody efficacy was 91% among subjects in the first or second trimester at recruitment. ► The corresponding vaccine effectiveness was estimated to be 79%.