BACKGROUND In a previous study, we found that monovalent acellular pertussis (aP) vaccine at birth and 1 month achieves higher IgG antibody (Ab) levels to pertussis toxoid (PT), filamentous hemagglutinin (FHA) and pertactin by 8 weeks, when compared with controls. Here, we report antibody and cell-mediated immune responses to 4 years of age. METHODS IgG Ab to PT, filamentous hemagglutinin and pertactin, diphtheria (D) and tetanus (T) was measured in the 3 groups (aP vaccine at birth and 1 month, aP birth only and no aP) at 2 years of age and before and after DTaP-inactivated polio vaccine (DTaP-IPV) at 4 years of age. Cell-mediated immune responses to pertussis vaccine antigens were measured at 2 years of age. Adverse events following DTaP-IPV were recorded. RESULTS Of 74 subjects, 52 (70%) were available for follow up. Overall, 11 (21%) had detectable PT IgG at 2 years, decreasing to 10% before 4-year-old booster compared with 100% at 8 months of age. After the 4-year booster, pertussis antigen IgG levels were similar, but there was a trend to lower PT IgG levels in birth aP infants (geometric mean concentrations: 28.7 EI.U/mL) compared with controls (geometric mean concentrations: 53.6 EI.U/mL). The cytokine responses to pertussis antigen stimulation were higher in aP recipients at 2 years of age. There was no difference in injection site reactions among groups following the DTaP-IPV booster at 4 years of age. CONCLUSIONS In the longest reported follow-up of infants who received aP vaccine at birth, we found a trend to lower PT IgG antibodies post booster compared with receipt of first dose of aP-containing vaccine at 8 weeks of age. Short- and long-term antibody responses with and without prior maternal pertussis vaccination are crucial for further evaluation of this strategy for preventing severe early pertussis.