Hib antibody responses in infants following diphtheria, tetanus, acellular pertussis, and conjugated Haemophilus influenzae type b (Hib) combination vaccines with decreasing amounts of tetanus toxoid.

BACKGROUND: While combination vaccines have contributed to improved vaccine uptake rates in children, studies have documented varied immunogenicity to specific vaccine components. We studied whether varying the amount of tetanus toxoid (TT) in a DTaP and Hib combination vaccine would result in immunogenicity comparable with separate, concurrent administration.

METHODS: We evaluated the immunogenicity of Massachusetts Biologic Laboratories (MBL) diphtheria, tetanus, and acellular pertussis (mDTaP) vaccine combined with tetanus-conjugated MBL Haemophilus influenzae type b vaccine (mHib) in a single injection (DTaPH). We compared four DTaPH vaccines containing varying concentrations of TT. We also evaluated the immune response to the DTaP vaccine manufactured by Connaught Laboratories (now known as Sanofi Pasteur) given with mHib and with Wyeth Hib-CRM197 (HbOC) as separate injections. Vaccines were administered to 240 healthy infants at 2, 4, and 6 months of age, and blood specimens for antibody determination were obtained before each immunization and one month after the third immunization.

RESULTS: We found no significant differences in immune response to the vaccines between the four DTaPH groups. Hib antibody responses were similar in the mHib and the HbOC groups but significantly lower in the DTaPH groups, as measured by Chinese Hamster Ovary (CHO) cell neutralization titers and filamentous hemagglutinin antigen (FHA) geometric mean concentrations (GMC) of anti-Hib antibodies. There were no significant differences between the groups in pertussis or tetanus toxoid antibody levels.

CONCLUSION: Reducing tetanus toxoid amounts did not produce comparable immunogenicity for Hib. The nature of the interaction between immune responses to DTaPH components should be explored further to enable the development of better Hib-containing combination vaccines.