THE IMPACT OF VACCINATION AGAINST INVASIVE HAEMOPHILUS-INFLUENZAE TYPE-B DISEASE IN THE SYDNEY REGION

Objective: To evaluate the incidence of invasive Haemophilus influenza e type b (Hib) disease relative to rates of Hib vaccination in a well defined population. Design and subjects: Cases of invasive Hib disease were identified by active laboratory surveillance for the period 1989 -1994, and retrospectively for 1985-1987. Vaccination rates were deter mined by telephone interview of families with children aged 0-4 years, identified in a random telephone directory sample of 4000 households. The receipt and time of vaccination were validated from general pract itioner records for a 50% subsample of children. Setting: Sydney Stati stical Division, with a population of 263 758 children aged 0-4 years in 1990. Results: Hib vaccination rates were relatively low before the introduction of government-funded vaccination programs in May 1993, e specially for children under 18 months for whom multiple doses are req uired. Rates rose from fewer than 9% (95% CI, 4%-13%) in May 1993 to 4 8% (CI, 40%-56%) in August 1993 for children under 18 months, and from 31% (CI, 26%-36%) to 45% (CI, 40%-51%) for children aged 19-60 months . The age-specific incidence of Hib disease was inversely related to t he vaccination rate. Forecasting of Hib disease incidence by the Box-J enkins method showed that from September 1993, when about a 50% vaccin e uptake was achieved in the eligible age group, overall incidence was substantially lower than expected. Conclusions: These data provide go od evidence that the decrease in Hib disease incidence in 1993-1994 is an effect of vaccination, and not annual or seasonal variation. The i mpact of Hib vaccination appears to have been greater than would be ex pected from protection of vaccinated children alone. Invasive Hib dise ase is likely soon to become a rare cause of serious childhood infecti on in Australia.