Persistence of antibody and booster responses to reimmunization with Haemophilus influenzae type b polysaccharide and polysaccharide diphtheria toxoid conjugate vaccines in children initially immunized at 15 to 24 months of age

To evaluate the persistence of antibody after Haemophilus influenzae type b polysaccharide vaccine (PRP) and H influenzae type b polysaccharide diphtheria toxoid conjugate vaccine (PRP-D), a group of 141 infants initially immunized between 15 and 24 months of age were studied 1 year later. One month after immunization with PRP, the man anti-PRP antibody level was 0.27 microgram/mL and 1 year later was 0.29 microgram/mL (not significant). In the group immunized with PRP-D, the levels were 1.34 micrograms/mL and 1.20 micrograms/mL (not significant), respectively. To evaluate immunogenicity and safety of a booster immunization 1 year after initial vaccination, subjects were randomly assigned to receive saline, PRP, or PRP-D. In addition, 73 age-matched previously unimmunized subjects were vaccinated with PRP or PRP-D. In all groups, adverse reactions were minor and resolved by 48 hours. Subjects receiving booster immunization with PRP or PRP-D had significantly greater antibody responses than children of the same age receiving their first dose of vaccine. The highest antibody levels were achieved in children initially immunized with PRP-D, regardless of whether the booster vaccine was PRP (112.8 micrograms/mL) or PRP-D (122.0 micrograms/mL) (not significant). Antibody levels after booster vaccine were significantly lower in those initially given PRP compared with those initially given PRP-D but significantly higher than in age-matched previously unimmunized control subjects (PRP booster 3.16 micrograms/mL vs control of 0.62 microgram/mL [P less than .05]; PRP-D booster 12.31 micrograms/mL vs control 2.31 micrograms/mL [P less than .01]).