Decrease in effectiveness of routine surveillance of Haemophilus influenzae disease after introduction of conjugate vaccine: comparison of routine reporting with active surveillance system

In October 1992 routine immunisation with Haemophilus influenzae type b con jugate vaccine was introduced in the United Kingdom, and the incidence of d isease was subsequently reported to have decreased 15-fold.(1) The surveill ance systems in place were primarily routine and were known to underestimat e the burden of invasive H influenzae disease.(2) This study aimed to deter mine whether underreporting continued after introduction of the conjugate v accine, and how this might affect the reported success of the vaccine. Resu lts of routine surveillance were compared with active surveillance for inva sive H influenzae disease in the West Midlands health region of England.