Determinants of persistent spread of HIV in HCV-infected populations of injecting drug users.

Hepatitis C virus (HCV) and human immunodeficiency virus (HIV) are both transmitted through populations of injecting drug users (IDU) by the sharing of contaminated syringes. Prevalence of HCV is high in most IDU populations, whereas HIV prevalence varies considerably across populations. Understanding the dynamics of these interacting infections may allow us to use HCV prevalence as an indicator for the risk of persistent spread of HIV. We developed a mathematical model that describes the spread of both HCV and HIV in an IDU population. The model allows for HCV-HIV co-infection and increased disease related mortality for both infections. Using this model we investigated how HIV and HCV prevalence both depend on level and heterogeneity of injecting risk behaviour, and how HIV and HCV prevalence are related. To gain knowledge of actual risk behaviour we analysed data from the Amsterdam Cohort Study (ACS) of drug users. We find that there is a threshold HCV prevalence at which HIV can invade into an IDU population; below threshold HIV cannot spread. This threshold depends strongly on heterogeneity of risk behaviour in the population, as well as on whether sharing is more likely to occur within or between risk behaviour groups. We find that our model agrees with the observed relationship between HCV and HIV prevalence as described by Vickerman et al. (2010), when in addition to risk heterogeneity as fitted from the ACS, we also assume that most contacts (>90%) occur amongst IDU of the same risk level (assortative mixing). We conclude that HCV prevalence can be used as an indicator of risk for successful HIV introduction into an IDU population. However, information on risk heterogeneity is required for determining this risk, and also for designing effective prevention strategies.