The impact of vaccination against invasive Haemophilus influenzae type b disease in the Sydney region

OBJECTIVE: To evaluate the incidence of invasive Haemophilus influenzae 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 determined 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 practitioner records for a 50% subsample of children. SETTING: Sydney Statistical 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, especially for children under 18 months for whom multiple doses are required. Rates rose from fewer than 9% (95% CI, 4%-13%) in May 1993 to 48% (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 the vaccination rate. Forecasting of Hib disease incidence by the Box-Jenkins method showed that from September 1993, when about a 50% vaccine uptake was achieved in the eligible age group, overall incidence was substantially lower than expected. CONCLUSIONS: These data provide good evidence that the decrease in Hib disease incidence in 1993-1994 is an effect of vaccination, and not annual or seasonal variation. The impact of Hib vaccination appears to have been greater than would be expected from protection of vaccinated children alone. Invasive Hib disease is likely soon to become a rare cause of serious childhood infection in Australia.