Long-term immunogenicity assessment of a DTaP-IPV//PRP-T vaccine given at 2, 4, 6 and 18-19 months of age, and immunogenicity and safety of a DTaP-IPV vaccine given as a booster dose at 4 to 6 years of age in Thai children.

Booster vaccination of infants aims to further reduce the burden of childhood infectious diseases. This study assessed the antibody persistence induced by a primary series vaccination at 2, 4, 6 months of age and a first booster at 18-19 months of age with a pentavalent diphtheria, tetanus, acellular pertussis, inactivated poliovirus, Haemophilus influenzae type b combined vaccine (DTaP-IPV//PRP-T) in 4-6 year-old Thai children (N=123). The safety and immunogenicity of a tetravalent acellular pertussis combined vaccine (containing the same DTaP-IPV antigens as the previous vaccine) given as a second booster at 4 to 6 years of age was also evaluated. Seroprotective antibody levels against diphtheria (> or = 0.01 IU/ml), tetanus (> or = 0.10 IU/ml), and polioviruses (> or = 8 1/dil) were maintained 4-6 years after primary-vaccination and first booster by > or = 92.7% of children, and anti-pertussis antibodies > or = 5 EU/ml were observed in the majority of children. The second booster with DTaP-IPV elicited a strong response for all antigens. GMT or GMC ratios for all antigens at the pre- and post-booster samples were from 4.7 to 52.5. Primary vaccination at 2, 4, 6 and a booster at 18-19 months of age with the DTaP-IPV//PRP-T vaccine induced satisfactory antibody persistence at 4-6 years of age. A second booster with DTaP-IPV induced a strong immune response and was well tolerated.