This study compared the levels of immunogenicity and safety of diphtheria-tetanus toxoid-five-component acellular pertussis (DTaP 5 ), inactivated poliovirus (IPV), and Haemophilus influenzae type b (Hib) (DTaP 5 -IPV-Hib) and DTaP 3 -IPV/Hib vaccines for study participants 3, 5, and 12 months of age. Post-dose 3 noninferiority criteria comparing DTaP 5 -IPV-Hib to DTaP 3 -IPV/ Hib using rates of seroprotection were demonstrated against diphtheria, tetanus, and polio types 1 to 3, but not for polyribosylribitol phosphate (PRP). While PRP did not meet noninferiority criteria, the seroprotection rate and geometric mean concentration (GMC) were high, indicating a clinically robust immune response. GMCs or titers for other antigens (including pertussis) and the safety profiles were generally similar between groups. Fully liquid DTaP 5 -IPV-Hib can be administered using the 3-, 5-, and 12-month vaccination schedule. (This study has been registered at ClinicalTrials.gov under registration no. NCT00287092.) D iphtheria-tetanus toxoid-five-component acellular pertussis (DTaP 5 ), inactivated poliovirus (IPV), and Haemophilus influenzae type b (Hib) (DTaP 5 -IPV-Hib) vaccine (Pediacel; Sanofi Pasteur Limited, Toronto, Ontario, Canada) is a fully liquid combination vaccine for primary and booster vaccination of infants and toddlers against infectious diseases caused by Clostridium tetani, Corynebacterium diphtheriae, Bordetella pertussis, Hib, and poliovirus types 1, 2, and 3. This licensed pentavalent vaccine comprises a 5-component acellular pertussis vaccine, adsorbed diphtheria and tetanus toxoids, inactivated poliomyelitis vaccine (IPV) grown in Vero cells, and a purified polyribosylribitol phosphate (PRP) capsular polysaccharide of Hib conjugated to tetanus toxoid. As a fully liquid formulation, DTaP 5 -IPV-Hib prevents dosing errors that can occur during reconstitution of vaccine components and may be more convenient for health care providers.The safety and immunogenicity of DTaP 5 -IPV-Hib using 3-dose primary vaccination schedules have previously been demonstrated in clinical studies in infants (1-8) and toddlers (4, 9-11). This is the first study to have evaluated the 3-, 5-, and 12-month schedule employed in some countries, particularly in Europe, and to have compared the safety and immunogenicity of (12). A modified double-blind design was utilized since the comparator vaccine (DTaP 3 -IPV/Hib) required reconstitution; an unblinded study team member prepared and administered study vaccines but was not involved in data collection. Parents/guardians were kept blinded to the identity of the study vaccine administered. The study complied with the Declaration of Helsinki. The study protocol and informed consent forms were approved by study site ethics committees. The participant parent(s) or legal guardian(s) provided written consent prior to study-specific procedures. The manuscript was prepared according to the Uniform Requirements for Manuscripts Submitted to Biomedical Journals guidelines.Participants. Partici...