The impact of Hib conjugate vaccine on the epidemiology of invasive Haemophilus influenzae disease in the West Midlands and the effect of deprivation and other environmental risk factors : an ecological study, 1990-1994
Objective: To describe the epidemiology of invasive Haemophilus influenzae (HI) disease, and to explore the relationship between socioeconomic disadvantage and invasive HI disease in the two years immediately before (October 1990 to September 1992), and following (October 1992 to September 1994) the introduction of HI type b (Hib) conjugate vaccine. Design: Multiple sources of case ascertainment were used to identify children with invasive HI disease in the West Midlands Health Region (WMHR) and compile a case register. An ecological study examined socioeconomic disadvantage using selected socioeconomic census data for enumeration districts, and the Townsend index of material deprivation. Setting: WMHR, England from October 1990 to September 1994. Subjects: Children under 5 years of age with invasive HI present on the West Midlands invasive HI case register (HICARE). Results : The incidence of invasive HI disease in the WMHR fell from 28.3/105 (95% CI=24.5 to 32.6) children <5 years of age in the pre-conjugate vaccine period to 5.4/105 (3.8 to 7.4) after the vaccine had been introduced. Nine vaccine failures were identified. Those aged 6-11 months were most at risk in the pre-conjugate vaccine period (OR=2.69,95% CI=1.14 to 6.55). In the post-conjugate vaccine period those aged 24-35 months were at most risk (OR=2.64,1.16 to 5.94). More cases of meningitis were identified in girls than boys but the difference was not statistically significant. Significantly more cases of invasive HI seen in children of South Asian origin aged less than 12 months than in non-South Asian children (OR=2.88,1.07 to 7.86). Children from affluent areas were at significantly reduced risk of disease compared to children from the more deprived areas (OR=0.73,0.54 to 0.99). In the second period of the study a number of significant risk factors were identified. Children living in areas containing predominantly rented accommodation (p=0.0025), mobile populations (p=0.013) and a low density of children aged <5 years per km2 (p=0.039) were at increased risk of invasive HI disease. Following meningitis, children from deprived areas were more likely to suffer sensorineural hearing loss (p=0.035). Case fatality was higher in the second period of the study, although not significantly so (OR=3.64,0.88 to 15.05). Conclusion: This four year study provides the first detailed account of the relationship between deprivation, , socioeconomic risk factors, ethnic group, and invasive HI disease in Britain. The incidence of the disease fell dramatically in the post-conjugate vaccine period. A number of socioeconomic risk factors were identified in the second period of the study. The data suggest that although Hib conjugate vaccine has greatly reduced the incidence of disease, children from deprived areas remain at greater risk than children in more affluent areas. They also indicate that the disease is still responsible for considerable sequelae and mortality.