To evaluate the immunogenicity, persistence of antibodies and reactogenicity of GSK Biologicals' DTPa (INFANRIX) and dTpa (BOOSTRIX), when administered to subjects 18-20 months old, compared with not giving a booster DTP vaccine at 18-20 months. Study double blinded for the two DTP vaccines and single blinded for the control arm.

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.

Patient-level data for this study will be made available through www.clinicalstudydatarequest.com following the timelines and process described on this site.