Elevated levels of maternal anti-tetanus toxin antibodies do not suppress the immune response to a Haemophilus influenzae type b polyribosylphosphate-tetanus toxoid conjugate vaccine

Reported are the effects of elevated levels of anti-tetanus antibodies on t he safety and immune response to a Haemophilus influenzae type b polyribosy lphosphate (PRP)-tetanus toroid conjugate (PRP-T) vaccine. A group of Thai infants (n = 177) born to women immunized against tetanus during pregnancy were vaccinated with either a combined diphtheria-tetanus-pertussis (DTP) P RP-T vaccine or DTP and a PRP-conjugate vaccine using Neisseria meningitidi s group B outer-membrane proteins as a carrier (PedVax HIB). Although mast infants possessed high titres (>1 IU/ml) of anti-tetanus antibodies, the DT P-PRP-T combined vaccine engendered an excellent antibody response to all v accine components. In both Vaccine groups >98% of infants attained anti-PRP antibody titres greater than or equal to 0.15 mu g/ml. The geometric mean anti-PRP antibody titres were 5.41 mu g/ml and 2.1 mu g/ml for infants immu nized with three doses of PRP-T versus two doses of PedVax HIB vaccines, re spectively (P < 0.005). Similarly, the proportion of infants who achieved t itres greater than or equal to 1 mu g/ml was higher in the PRP-T group (87. 8%) than in the group immunized with PedVax HIB (74.2%) (P = 0.036). A subg roup analysis showed that there was no significant difference in the anti-P RP antibody response far infants exhibiting either <1 IU of anti-tetanus an tibody per millilitre or greater than or equal to 1 IU/ml at baseline. Thes e finding indicate that pre-existing anti-carrier antibody does not diminis h the immune response to the PRP moiety. Ail infants possessed protective l evels of anti-D and anti-T antibody levels after immunization.