Children in whom invasive Haemophilus influenzae type b disease develops despite previous immunization with conventional type b polysaccharide vaccine (polyribosylribitol phosphate [PRP]) have been reported to have impaired serum anticapsular antibody responses following recovery from H influenzae type b disease.1,2 Recently, these "vaccine failure" patients also have been found to have impaired antibody responses to reimmunization with PRP vaccine.3 The reasons are not understood because most of the children have normal serum concentrations of immunoglobulins, including IgG2.1-4 They also are more than 3 years of age at the time of reimmunization, an age group in which most healthy children respond to PRP vaccine.