Antibodies to Haemophilus influenzae serotype b in the Netherlands a few years after the introduction of routine vaccination

We assessed antibodies to the capsular polysaccharide of Haemophilus influe nzae type b (HibPS) in the Dutch population a few years after a mass vaccin ation against H. influenzae (Hib) was begun. We observed sharp declines in the geometric mean titer (GMT) and the prevalence of HibPS antibodies at le vels of less than or equal to0.15 mug/mL in children who had received 4 dos es of vaccine: from 8.65 mug/mL (prevalence, 99.4%) after 0-2 months to 0.8 mug/mL (prevalence, 83.3%) after 27-29 months. In adult groups, both the p revalence of HibPS antibodies and the GMT declined significantly with incre asing age but remained high (prevalence, greater than or equal to 83.7%; GM T, 0.73 greater than or equal to mug/mL). We conclude that the overall immu nity in the Dutch population seems satisfactory. We draw our conclusions fr om the current serosurveillance study and from the sharp decline in invasiv e Hib disease noted after the introduction of vaccination. The key question s for the future are (1) whether Hib and cross-reacting organisms will circ ulate sufficiently to provide natural reexposure, and (2) how long memory i mmunity will persist after vaccination without reexposure.