To evaluate the safety and immunogenicity of diphtheria-tetanus toxoids-acellular pertussis ( DTPa ) - hepatitis B ( HepB ) combination vaccine given at 2 , 4 and 6 months of age compared with monovalent HepB vaccine given at birth , 1 month and 6 months of age and DTPa vaccine given at 2 , 4 and 6 months of age .
