RESUMEN
OBJECTIVE: Invasive infections by Haemophilus influenzae type b (Hib) in children are increasing in Spain. Moreover, they are becoming more resistant to antibiotics. Currently, effective vaccines against Hib are available and their inclusion in the routine schedule of immunizations is discussed. Knowledge of the childhood serology appears to be useful in decision making. PATIENTS AND METHODS: Two hundred and thirty-five normal children, between 1 month and 15 years of age, were studied. Thirty-six adults sera were also included as controls. Polysaccharide of Hib was conjugated with tyramine and used to coat polystyrene microtitration plates. The determination of antibodies was performed by ELISA and the results are expressed in optical density units. The mean level of antibodies present in 6 sera from a patient with Bruton syndrome (17 uu), just before degammaglobulinotherapy, was considered as the limit of protection. RESULTS: The antibody titer correlated with age (p < 0.001). The levels in children younger than 3 years (25.7 +/- 32.5) were lower than in older children (p < 0.001). Of the children younger than 3 years of age, 53% did not reach the protective level of antibodies, compared to 15.2% of those between 3-6 years and 22.7% of children between 6 and 14 years. Whereas the mean antibody level increased with age in positive sera, the percentage of seropositive children decreased after 3 years of age. CONCLUSIONS: The number of children spontaneously protected against Hib is low (47%) until 3 years of age. Nevertheless, this number increases rapidly after this age. Immunization must be restricted to this age group.