Immunogenicity, Antibody Persistence and Safety of GSK Biologicals' DTPa (INFANRIX) and dTpa (BOOSTRIX) Vaccines.

Inclusion Criteria: * Male or female children between and including 18 -20 months of age at the time of the vaccination. * Completed a primary vaccination course with DTPa (INFANRIX) vaccine at 2, 4, and 6 months. * Written informed consent obtained before study entry from the parents or guardians of the subject. * Free of obvious health problems as established by medical history and clinical examination before entering into the study. Exclusion Criteria: * Use of any investigational or non-registered drug or vaccine other than the study vaccine(s) within 30 days preceding the administration of the study vaccine dose, or planned use during the study period. * Evidence of previous or intercurrent diphtheria, tetanus, or pertussis disease, or of vaccination against any of these diseases since completion of the primary course of DTPa (INFANRIX) vaccine.