Predicting the impact of treatment as prevention for people who inject drugs

Treatment as prevention among injecting drug users; extrapolating from the Amsterdam cohort study.

de Vos AS, Prins M, Coutinho RA, van der Helm JJ, Kretzschmar ME. AIDS 2014 Jan 7. [Epub ahead of print]

Objective: To determine the potential of treatment as prevention for reducing HIV incidence among injecting drug users (IDU).

Methods: Transmission dynamics of HIV as influenced by cART uptake and demographic changes were studied using an individual-based model. Parameters were based on data of the Amsterdam Cohort Study, and counterfactual treatment scenarios were examined for this city. Demography of the modeled population was also varied to allow for more general conclusions.

Results: We estimated that over the complete HIV epidemic among IDU in Amsterdam the historic use of cART has led to only 2% less incidence. As individuals were treated from low CD4-counts, their decreased infectiousness was offset by increased infectious lifetime. Large reduction in incidence could result from a test and immediate treat strategy, with elimination of HIV occurring when the average time from infection to starting treatment was less than 2 months. However, substantial proportions of new infections were prevented only if the test and treat intervention was implemented within the first few years after HIV-epidemic onset, especially for a declining IDU population. Ignoring heterogeneity in risk-behavior led to overly optimistic expectations of the prevention effects of treatment. In general, treatment led to much greater reduction in incidence compared with stopping HIV+ IDU from lending out syringes.

Conclusion: A test and immediate treat strategy for HIV among IDU could lead to great reductions in incidence. To fully eliminate the spread of HIV, treatment as prevention should be combined with other interventions, with behavioral intervention directed at those not yet HIV infected.

Abstract access  [1]

Editor’s notes: Treatment as prevention has been shown to substantially reduce the risk of sexual HIV transmission, and modelling has been used to estimate the impact of treatment as prevention in epidemics driven by sexual spread. Much less attention has been paid to the potential for treatment as prevention to reduce transmission amongst people who inject drugs, who account for 10% of people living with HIV globally. In this modelling study, the authors use a mathematical model of HIV transmission amongst people who inject drugs in Amsterdam to estimate the impact that treatment has had in this setting. The authors then estimate the impact that different testing and treatment strategies could have had in this setting and in other settings with increasing or stable populations of people who inject drugs. Their findings suggest that treatment has had little impact on incidence among people who inject drugs in Amsterdam.  This was partly due to late treatment, declining HIV prevalence and a declining population of people who inject drugs. They find that more frequent testing, coupled with earlier treatment could have a large impact if it is introduced soon after the introduction of HIV into populations of people who inject drugs, particularly if the population is stable or increasing in size. This could be relevant in places where injecting drug use is becoming more common, or in settings where HIV has only recently begun to be spread through injecting drug use. The authors do highlight however, that treatment as prevention alone is unlikely to lead to elimination of HIV, and that harm reduction strategies remain important.     

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