Clinical and immunological risk factors associated with Haemophilus influenzae type b conjugate vaccine failure in childhood

Haemophilus influenzae type b (Hib) conjugate vaccines have proved extremel y efficacious in healthy children. True Hib vaccine failures are rare. Hib conjugate vaccines were introduced for routine immunization in the United K ingdom and the Republic of Ireland in 1992, Coincident with this, active pr ospective and national surveillance via pediatricians, microbiologists, and public health physicians was commenced to assess the clinical and immunolo gical factors associated with vaccine failure. During the 6 years of the st udy, 115 children with true vaccine failure were reported. Of the children who were vaccinated before 12 months of age, a clinical risk factor was det ected in 20%, an immunological deficiency was detected in 30%, and one or b oth were detected in 44%. Children who were vaccinated after 12 months of a ge were more likely to have one or both factors (67%). Thirty percent (33 o f 105) of children with true vaccine failure had a low Hib antibody respons e (concentration, <1.0 <mu>g/mL) after disease, but the majority then respo nded to a further dose of Hib vaccine. Children who develop Hib disease des pite vaccination deserve further clinical and immunological evaluation.