Haemophilus influenzae type b is the most common cause of bacterial meningitis in children in the United States. Antibody to the capsular polysaccharide of this organism, polyribosylribitol-phosphate (PRP), is important in protecting against invasive disease. In 1985, H influenzae type b vaccine containing only PRP was licensed in the United States for use in children 24 to 59 months of age.

Studies conducted in the United States and Finland have shown that PRP vaccine is safe and immunogenic in children older than 18 months of age.1,2 In Finland, an efficacy of approximately 80% was obtained when PRP vaccine was used to prevent invasive disease caused by H influenzae type b in children older than 24 months of age.3 The immunogenicity of the PRP vaccine in children 18 to 23 months of age, however, was not clearly established. Reported rates of seroconversion after receipt of a single dose of PRP vaccine ranged from 20% to 75%1.3-5

The purpose of our studies was to evaluate the immunogenicity y of PRP vaccines and of conjugate vaccine (PRP-Neisseria meningitidis outer membrane protein complex [PRP-OMPC]) in children from the Los Angeles area.

First, we conducted a randomized, prospective study to compare the immunogenicity of the PRP vaccines produced by three manufacturers. A total of 147 healthy children between the ages of 18 and 58 months were recruited from Childrens Hospital of Los Angeles and Kaiser Foundation Hospital, Los Angeles, to receive, according to a prepared randomization schedule, 25 µg of the following PRP vaccines: Hib-Immune, lot 184-669, 190-654; Hib-Vax, lot 7L91071, 7D91043; or b-CAPSA I, lot M087BD, M068BC.

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