Immunization with Haemophilus influenzae type b polysaccharide vaccine at 18 and 24 months of age: evidence of decreased immunogenicity.

In order to assess the immunogenicity of a two-dose regimen of Haemophilus influenzae type b polysaccharide vaccine, the immune response to vaccine given at both 18 and 24 months of age was compared to the response to a single dose at 24 months of age. Following immunization at 24 months of age, the geometric mean antibody concentration of children previously immunized at 18 months (0.53 micrograms/ml) was significantly lower than that of children who received a single dose of vaccine at 24 months (1.03 micrograms/ml; p = 0.03). A four-fold rise in antibody concentration was demonstrated in 31% of children who had received two immunizations and 53% of children immunized for the first time at 24 months of age (p = 0.02). These results suggest that the administration of a dose of plain polysaccharide vaccine at 18 months of age blunted the response to reimmunization at 24 months of age. A subgroup of children who failed to respond to one or two doses of plain polysaccharide vaccine were immunized with polysaccharide-diphtheria toxoid conjugate vaccine. The majority of these children developed a significant rise in antibody concentration in response to conjugate vaccine.