One hundred forty-one children of 5 to 59 months of age were immunized with a single intramuscular dose of 0.67, 3.3, 17, or 67 μg polyribophosphate (PRP), the capsular antigen ofHemophilus influenzae, type b. The immunizations were well tolerated, particularly at doses of .67 to 17 μg. Antibody activity was measured by radioactive antigen binding, using3H-labelled PRP. Doses of 3.3 and 17 μg produced significant antibody rises in nearly 90% of recipients; 0.67 and 67 μg in approximately half. The geometric mean titers were similar at three and six weeks after immunization and were greater with the middle doses.

The net antibody increase in responding children was strongly age dependent, but was not related to the preimmunization antibody concentration. Rises in serum bactericidal activity against H. influenzae type b generally accompanied rises in antibody concentration as measured by the antigen-binding assay.

A recently developed Haemophilus influenzae type b capsular polysaccharide vaccine was given to 48 977 children 3 months to 5 years of age; an equal number of children receiving group A meningococcal vaccine served as controls. The protection as well as serum antibody response was strongly age dependent. Among children who had received the H. influenzae type b vaccine when 18 months of age or older, there were no cases of bacteremic disease caused by H. influenzaetype b in the first year after vaccination. At the same time 11 such cases were seen in the control group of the same age, a highly significant difference. In the second year after vaccination two cases occurred in the H. influenzae type b-vaccinated group, five in the meningococcal-group A vaccinated group. No protection was seen among children who had been younger than 18 months when vaccinated, even if they received a booster dose of the vaccine.

The serum antibody response to the H. influenzae type b polysaccharide, measured by radioimmunoassay, was poor in children below 18 months of age and good in those above it. No effect of the vaccine could be seen on the nasopharyngeal carriage of H. influenzae type b, which was approximately 6% in this age group. Adverse effects of the vaccine were mild.

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