Antibody response to Haemophilus influenzae type b tetanus conjugate vaccine with two doses given at 3 and 5 months of age.

BACKGROUND In developed countries, the use of Hib conjugate vaccines has led to the near disappearance of invasive Hib disease, but costs have limited its use in developing countries. In order to identify more economical vaccination schedules, we carried out a trial to evaluate the immunogenicity of an alternative two-dose PRP-T regimen, based on a previous report in which carrier priming could be obtained with prior diphtheria-tetanus-pertussis (DTP) vaccination. METHODS Healthy infants were enrolled to receive the PRP-T given at 3 and 5 months of age, with DTP vaccination given at 2, 4 and 6 months of age. Serum specimens were obtained at 3, 6 and 15 months of age. IgG anti-Hib titer determination was performed using enzyme-linked immunosorbent assay to evaluate serologic response and its duration. RESULTS One-hundred and seventeen infants were enrolled. The geometric mean titer (GMT) of antibody to PRP was low in the pre-immunization samples (0.13 mg/mL), achieving high values after two doses of PRP-T (27.42 mg/mL), with all titers over 1 mg/mL; the GMT at 15 months was 5.45 mg/mL; 94.6% of infants had serologic responses after the two doses of vaccination, with average intervals of 27 and 22 days between DTP and PRP-T first-to-first and second-to-second administrations, respectively. However, these intervals were 11 and 3 days for infants who did not have serologic responses (P50.0013 and 0.0030, respectively). CONCLUSIONS These results indicate that two doses of PRP-T can induce high antibody titers using the proposed schedule; moreover, the GMT assessed at 15 months of age was also protective. The enhanced immune response observed in the study could be explained by the previous administration of the DTP vaccine, since the longer the interval between DTP and PRP-T, the better the response to Hib vaccine. The PRP-T vaccine given at 3 and 5 months of age may be an economical alternative to the current proposed schedule, which could make the introduction of Hib vaccination in developing countries more feasible, considering the relatively high cost of this vaccine.