Serum antibodies against haemophilus influenzae type b and tetanus at 2.5 years of age: a follow-up of 2 different regimens of infant vaccination.

In 1990-91 we compared 2 Haemophilus influenzae type b (Hib) conjugate vaccines and 2 different regimens of tetanus vaccination, by vaccinating 142 Swedish infants at ages 3, 5, and 12 months, with either PRP-T (the capsular polysaccharide of Hib conjugated to tetanus toxoid) + D (diphtheria toxoid), or with PRP-OMP (PRP conjugated to an outer membrane complex of meningococcus group B) + DT (diphtheria-tetanus toxoid). In this follow-up, antibodies against Hib and tetanus were analyzed in sera from 133 of the children at the age of 2.5 years. Hib antibodies (> or = 0.06 micrograms/ml) were found in 99% of the children of both groups, but 93% of the PRP-T vaccinees had maintained Hib antibodies > or = 0.15 micrograms/ml, as compared with 80% of the PRP-OMP vaccinees (p < 0.05). In 1992, the batch of PRP-OMP was reported to have questionable immunogenicity. Tetanus toxoid (T) antibodies (> or = 0.01 IU/ml) were found in all sera from both groups. All sera with T antibodies < 0.1 IU/ml showed tetanus toxin neutralizing activity. However, only 75% of children vaccinated with PRP-T had T antibodies > or = 0.1 IU/ml, as compared to 97% of children vaccinated with DT (p < 0.001). In conclusion, Hib and tetanus antibodies were well maintained 18 months after primary vaccination, also in the group vaccinated with the batch of PRP-OMP of somewhat low immunogenicity and in the group of infants receiving their primary tetanus vaccination only by the carrier protein of PRP-T.