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Considerable evidence indicates that both anticapsular antibody and immunologic memory play a role in immunity to Haemophilus influenzae type b (Hib) disease. The efficacy of memory (or antibody) cannot be expected to be 100%; therefore some individuals may develop invasive disease despite their having been naturally primed. The proportion of cases of H. influenzae type b disease with evidence of immunologic memory is related to both the efficacy of memory in preventing disease and the age-related prevalence of memory in the population. The task is to discern the relative contributions of antibody and memory in conferring protection and to determine the extent to which natural exposure and vaccination establish these two effector mechanisms.