Invasive Haemophilus influenzae disease: an ecological study of sociodemographic risk factors before and after the introduction of Hib conjugate vaccine.

This study examines the impact of H. influenzae type b (Hib) conjugate vaccine on sociodemographic risk factors for invasive H. influenzae disease in the 2 years before and immediately after the introduction of Hib conjugate vaccine. An ecological study design was used and cases were identified using active surveillance employing several surveillance systems. The study population comprised all children aged < 5 years resident in the West Midlands, an English health region, with laboratory confirmed invasive disease 2 years before (1990-1992) and 2 years after (1992-1994) the introduction of Hib conjugate vaccine. Selected sociodemographic variables derived from the UK census were obtained for all census enumeration districts in the region. Each variable was then ranked and divided into six categories. Linear associations between disease rates and sociodemographic variables were examined. Overall, there was a significant reduction in the incidence of invasive H. influenzae disease. In the pre-conjugate vaccine era there were trends of decreasing disease incidence with increasing child population density (p = 0.012) and total population density (p = 0.0023). In the post-conjugate vaccine period, total population density (p = 0.0275) remained significant and a trend of increasing disease incidence with increasing population mobility (p = 0.0012) was seen. Although Hib conjugate vaccine has resulted in a dramatic reduction in disease incidence changes in sociodemographic risk factors were identified in the post-conjugate vaccine period, particularly population mobility. Our results may have implications for current and future vaccine strategies.