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 67µg polyribophosphate (PRP), the capsular antigen of Hemophilus influenzae, type b. The immunizations were well tolerated, particularly at doses of .67 to 17µg. Antibody activity was measured by radioactive antigen binding, using 3H-labelled PRP. Doses of 3.3 and 17µg produced significant antibody rises in nearly 90% of recipients; 0.67 and 67µg 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.