Non-type b Haemophilus influenzae disease: clinical and epidemiologic characteristics in the Haemophilus influenzae type b vaccine era

Background. As a result of the decline in Haemophilus influenzae type b (Hi b) disease caused by the widespread use of conjugate vaccines, non-type b H . influenzae will become a more important cause of H. influenzae (Hi) disea se. Characterization of the clinical and epidemiologic features of non-b Hi disease is needed in the Hib vaccine era. Methods. A prospective active surveillance study of invasive Hi disease inv olving pediatricians in the United Kingdom and Republic of Ireland. For the first phase of the study (October 1, 1992, to October 31, 1995) pediatrici ans were asked to report any child who had invasive Hi disease and who had received Hib conjugate vaccine. For the second phase of the study (November 1, 1995. To December 31, 1998) pediatricians were asked to report any chil d with invasive Hi disease regardless of vaccination status. Results. During the study period 102 cases of invasive non-type b Hi diseas e and 106 cases of invasive Hib disease were reported in children who had b een fully vaccinated against Hib. Children with non-type b disease were you nger (16 vs. 22 months of age, P = 0.08), less likely to have meningitis an d epiglottitis (P less than or equal to 0.001) and more likely to have pneu monia and bacteremia (P less than or equal to 0.001) than children with typ e b disease. For the last 2 years of the study invasive Hi disease occurrin g in a fully vaccinated child was more likely to be caused by a non-b strai n than by a type b strain (58 vs. 38). In 1998 the incidence of non type-b Hi disease in all children <5 years of age in the UK was 1.3/100 000 as com pared with an incidence of Hib disease of 0.6/100 000. The majority (88%) o f non-b strains isolated in children were nontypable strains. Conclusions. Non-b Hi is a rare cause of disease in children, but in the Hi b vaccine era it has become more common than type b as a cause of Hi diseas e in fully vaccinated children.