A comparison between the force of infection estimates for blood-borne viruses in injecting drug user populations across the European Union: a modelling study

A number of studies have been conducted in injecting drug user (IDU) populations in Europe, in which the prevalence of human immunodeficiency virus (HIV), hepatitis B virus (HBV) and hepatitis C virus (HCV) was measured together with demographic and epidemiological information such as age and the age at first injection. A measure of the risk of becoming infected is the force of infection (FOI), defined as the per capita rate at which susceptible individuals acquire infection. The objective of this study was to estimate the FOI and its heterogeneity for HBV, HCV and HIV (where available) for IDU populations in a number of countries in Europe. Data were obtained from five countries: Belgium, the United Kingdom, Spain and Italy, and the Czech Republic, which provided two data sets. The model describes the prevalence of infection as a function of the FOI that may vary over time or duration of IDU. In addition to this, if two or more infections were being considered then a parameter describing the potential heterogeneity of the FOI within the IDU population was also estimated. The results here add to the growing evidence that new initiates to injecting are at an increased risk of blood-borne viral infection compared with more experienced IDUs. In addition, there is evidence of individual heterogeneity of FOI estimates within the overall IDU populations. This suggests that different proportions of individuals in each population are at increased risk of infection compared with the rest of the population. Future interventions should identify and target these individuals. Moreover, changes over time in individual heterogeneity estimates of IDU populations may provide an indicator for measuring intervention impacts.