Vaccination of pregnant women with a pertussis containing vaccine is a recommended strategy in some industrialized countries, to protect young infants from severe disease. One of the effects of the presence of high titers of passively acquired maternal antibodies in young infants is blunting of immune responses to infant vaccination. We present infant immune responses to a fourth pertussis containing vaccine dose at 15 months of age, as a follow-up of previously presented data. In a prospective cohort study, women were either vaccinated with an acellular pertussis vaccine (Boostrix(®)) during pregnancy (vaccine group) or received no vaccine (control group). All infants were vaccinated with Infanrix Hexa(®) according to the standard Belgian vaccination schedule (8/12/16 weeks, 15 months). We report results from blood samples collected before and 1 month after the fourth vaccine dose. Immunoglobulin G (IgG) antibodies against pertussis toxin (PT), filamentous hemagglutinin (FHA), pertactin (Prn), tetanus toxoid (TT) and diphtheria toxoid (DT) were measured using commercially available ELISA tests. Antibody levels were expressed in International Units per milliliter. Demographic characteristics were similar in the vaccine and control group. Before the fourth vaccine dose, significantly lower antibody titers were measured in the vaccine group compared to the control group for anti-Prn IgG (p=0.003) and anti-DT IgG (p=0.023), with a steep decay of antibody titers since post-primary vaccination. One month after the fourth dose, antibody titers were only significantly lower in the vaccine group for anti-PT IgG (p=0.006). For all antigens, there was a rise in antibody titer after the fourth vaccine dose. The present results indicate still a minor blunting effect 1 month after a fourth vaccine dose for anti-PT antibodies. However, a good humoral immune response on all measured antigens was elicited in both groups of children. The clinical significance of such blunting effect is yet unknown. Clinicaltrials.gov identifier: NCT01698346.