Vlahov, David

Zwahlen, Marcel

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Human immunodeficiency virus (HIV) prevention programmesfor injection drug users (IDU) have included elements such asoutreach education, testing, counselling, bleach distribution, andwider access to sterile needles through unrestricted pharmacyaccess and needle exchange programmes. Needle exchangeprogrammes work by providing sterile in exchange for con-taminated needles alongside outreach education and healthservice referrals. With both HIV and hepatitis viruses known to betransmitted by multiperson use of needles and syringes, provid-ing access to sterile needles for those who cannot or will notstop drug use would appear to be an important strategy. Anearly observational study of the effectiveness of needle exchangeprogrammes showed that harm reduction was associated withreduced rates of hepatitis B and C virus infections,1 and otherobservational studies have shown lower rates of HIV.2,3 However,subsequent work has shown that this is not always the case4 andthat rates of hepatitis C virus (HCV) in IDU have tended toremain elevated even in communities where HIV rates havedeclined.5 This discrepancy has been puzzling, and the modellingstudy by Murray and coworkers in this issue provides a con-tribution to advance our thinking about this.6Although both HIV and HCV share similarities in that drugusers are at risk through parenteral transmission and both arepredominantly chronic infections, there are important differ-ences between the two infections. First, the prevalence of infec-tion among drug users is higher for HCV than for HIV, indicatinga larger pool of HCV from which transmission can occur.Second, data from needlestick injuries suggest that the rate oftransmission per individual needle stick may be about 10 timeshigher in HCV than HIV infection.7 Third, indirect sharing anddrug preparation practices such as backloading and sharing ofthe cotton, cooker, and rinse water used to prepare drugs forinjection are more frequently associated with transmission ofHCV5,8 than HIV.9 One report indicating the possibility of trans-mission with straws used to inhale cocaine suggests an efficiencyof transmission for HCV that has not been reported to date forHIV.10Murray et al. suggest that these known or suspectedepidemiological differences did not fully explain the intriguingsituation in Australian injection drug users where the reportedHIV prevalence remained very low,11 but HCV prevalence wasabout 50% and comparable to that observed in other injectiondrug user populations with much higher HIV prevalence. Toexplain this situation they modelled virus transmission amonginjection drug users via ‘classic’ needle sharing in such a way asto handle both HCV and HIV while capturing the main differ-ences between the two. An interesting approach was chosen bymodelling drug sharing episodes among groups of a given size.The model allowed for calculating critical levels of needle sharingbelow which total infections would fall to a low prevalence.These levels were estimated to be 17 injection partners per yearfor HIV and 3 for HCV and were compared to 6 injectionpartners per year currently estimated in Australia.Mathematical propagation models such as this have a numberof uses. First, they can provide qualitative insights into therelevance of certain key parameters or structural aspects of thedynamics of propagation. The results of this study indicate thatbehavioural changes in needle sharing starting in the late 1980sin Australia might have been enough to limit the spread of HIVbut insufficient to substantially limit or reduce HCV prevalence.Second, modelling may provide the public health communitywith quantitative predictions of the possible effect of interventionstrategies to assist policymaking. Here, the model estimates the number and prevalence of HIV and HCV infections in theinjection drug user population of Australia (Figure 2) and thecritical level of needle sharing necessary for successfully reducingthese infections. The authors should be applauded for havingprovided estimates of the uncertainty of some of their results, afeature not always included in such modelling studies. How-ever, the uncertainty for HCV prevalence was considerable,with an inter quartile range from 20 to 50% for the year 2000,and these estimates were much more imprecise than those forHIV prevalence (Figure 2). It would have been important toknow which of the parameters and their uncertainty was prim-arily responsible for this substantial difference. No estimates onuncertainty of the critical levels of needle sharing were given,however. These would have enabled better judgement ofwhether the critical level of three partners per year can indeedbe expected to reduce HCV to low levels.As mathematical modelling studies are inevitably based onassumptions, one can always argue about how appropriatethese are, and whether they are unrealistic or too simplistic andtherefore hamper firm qualitative or quantitative conclusionsabout the reality. Here, the authors chose to build the model on© International Epidemiological Association 2003 Printed in Great Britain International Journal of Epidemiology 2003;32:715–716DOI: 10.1093/ije/dyg169Commentary: Modelling and understandingdifferences in human immunodeficiency andhepatitis C virus epidemiology in injectiondrug usersMarcel Zwahlen1 and David Vlahov21 Department of Social and Preventive Medicine, University Berne, Berne,Switzerland.2 Center for Urban Epidemiologic Studies, New York Academy of Medicine,New York, USA.parameters that reflect the average value over all injection drugusers in Australia. For example, the average number ofinjections per year was chosen to be 60, with a range of 30 to 90.This value was based on cross-sectional data on self-reportedneedle sharing within the last month and some model-specificequations that allowed this information to be related to thenumber of injections. Given the heterogeneity in drug con-sumption intensity between sporadic, regular, and heavily drugdependent injectors, this may be too strong a simplification.More complex compartment modelling would have been neces-sary to account for that. Additionally, per contact infectivity wasassumed to be 2% (0.3–3.3%) for HIV and 4% (1.2–10%) forHCV (Table 1), and constant after infection. However, not allHCV infected individuals remain persistently infected and contactinfectivity seems to be higher with higher levels of viral load12and can be expected to be much higher in acute viraemia afterinfection. Incorporating acute viraemia may be necessary toadequately explain the fact that HCV may be transmitted rapidlyin the first few years after initiation of injection use, or amongvery young injectors.11,13In conclusion, the US Public Health Service have issued anHIV Prevention Bulletin14 to warn injection drug users whocould not quit using drugs to use (and then discard) a newsterile needle each time they injected. While rates of HIV havedeclined, the persistent high rates of HCV and the concernabout transmission with even a few partners suggested in thismodelling exercise indicate that efforts to encourage single useand discard syringes remain important.AcknowledgementThe authors would like to thank Beat E Neuenschwander andNicola Low for helpful advice on modelling issues and usefulcomments.References1 Hagan H, Jarlais DC, Friedman SR et al. Reduced risk of hepatitis Band hepatitis C among injection drug users in the Tacoma syringeexchange program. Am J Public Health 1995;85:1531–37.2 Kaplan EH, Heimer R. HIV incidence among New Haven needleexchange participants: updated estimates from syringe tracking andtesting data. J Acquir Immune Defic Syndr Hum Retrovirol 1995;10:175–76.3 Des Jarlais DC, Marmor M, Paone D et al. HIV incidence amonginjecting drug users in New York City syringe-exchange programmes.Lancet 1996;348:987–91.4 Hagan H, McGough JP, Thiede H et al. Syringe exchange and risk ofinfection with hepatitis B and C viruses. Am J Epidemiol 1999;149:203–13.5 Diaz T, Des Jarlais DC, Vlahov D et al. Factors associated withprevalent hepatitis C: differences among young adult injection drugusers in lower and upper Manhattan, New York City. Am J PublicHealth 2001;91:23–30.6 Murray JM, Law MG, Gao Z and Kaldor JM. The impact of behaviouralchanges on the prevalence of human immunodeficiency virus andhepatitis C among injecting drug users. Int J Epidemiol 2003;32:708–14.7 MacDonald M, Crofts N, Kaldor J. Transmission of hepatitis C virus:rates, routes, and cofactors. Epidemiol Rev 1996;18:137–48.8 Thorpe LE, Ouellet LJ, Hershow R et al. Risk of hepatitis C virusinfection among young adult injection drug users who share injectionequipment. Am J Epidemiol 2002;155:645–53.9 Samuels JF, Vlahov D, Anthony JC et al. The practice of ‘frontloading’among intravenous drug users: association with HIV-antibody. AIDS1991;5:343.10 Conry-Cantilena C, VanRaden M, Gibble J et al. Routes of infection,viremia, and liver disease in blood donors found to have hepatitis Cvirus infection. New Engl J Med 1996;334:1691–96.11 van Beek I, Dwyer R, Dore GJ et al. Infection with HIV and hepatitisC virus among injecting drug users in a prevention setting:retrospective cohort study. BMJ 1998;317:433–37.12 Dore GJ, Kaldor JM, McCaughan GW. Systematic review of role ofpolymerase chain reaction in defining infectiousness among peopleinfected with hepatitis C virus. BMJ 1997;315:333–37.13 Garfein RS, Vlahov D, Galai N et al. Viral infections in short-terminjection drug users: the prevalence of the hepatitis C, hepatitis B,human immunodeficiency, and human T-lymphotropic viruses. Am JPublic Health 1996;86:655–61.14 United States Public Health Service. HIV Prevention bulletin: MedicalAdvice for Persons who Inject Illicit Drugs. 9–5–1997. Atlanta, USA,Centers for Disease Control and Prevention.716 INTERNATIONAL JOURNAL OF EPIDEMIOLOGY

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