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The safety and immunogenicity of TETRActHIB (a vaccine combining diphtheria and tetanus toxoids-pertussis vaccine (DTP) with Haemophilus influenzae type b (Hib) conjugate vaccine (polyribosyl ribitol phosphate conjugated to tetanus protein) (PRP-T)) was assessed in 131 Cape Town infants immunised at 6, 10 and 14 weeks of age. Serological responses to all component antigens were measured before the first dose and at 18 weeks of age. In addition, anti-PRP antibodies were measured at 9 and 18 months of age to determine long-term immunogenicity. The vaccine was well tolerated by infants and no significant side-effects were reported. Responses to Hib at 18 weeks of age were good in that most infants achieved a level of anti-PRP antibodies <". 0.15 μg/ml, indicative of short-term protection, and 70% achieved a level<". 1 μg/ml, indicative of long-term protection. The proportions of children with protective levels<". 0.15 μg/ml and<". 1 μg/ml were similar at 9 and 18 months of age, i.e. approximately 75% and 45%, respectively. Responses to tetanus and diphtheria toxoids were excellent and all infants achieved protective serological levels. Responses to pertussis were moderate in that approximately 65% achieved 'protective' serum levels of pertussis agglutinins, i.e. titres <". 320. In conclusion, this study has shown that the DTP /PRP-T vaccine is safe, immunogenic and well tolerated in infants immunised at 6, 10 and 14 weeks of age. TETRActHIB is therefore suitable for inclusion in the World Health Organisation Expanded Programme on Immunisation (WHO EPI) schedule.