Effectiveness of syringe exchange programs in reducing HIV risk behavior and HIV seroconversion among injecting drug users.
Drug addiction is a major risk factor for HIV infection for an estimated 3.4–5.5 million injecting drug users (IDU) in more than 125 countries [1–3]. In the USA, injecting drug use now accounts for nearly one-third of new AIDS cases [4] and for as many as three-quarters of annual incident HIV infections [5]. Behaviors associated with drug use that are responsible for HIV transmission include shared use of injection equipment and other drug paraphernalia, and unprotected vaginal and anal intercourse with an injecting drug user. Interventions that can reduce the prevalence of these behaviors are therefore a critical component of comprehensive AIDS prevention policy. Approaches to HIV risk reduction among IDU include drug abuse treatment, HIV testing and counseling programs, street-based outreach conducted by peer educators, individual and group counseling, community-level interventions to change IDU norms concerning safer injection and safer sex, and syringe exchange programs to provide IDU with sterile injection equipment. Syringe exchanges have been established widely in some developing, and a majority of the developed countries, [3] including the USA, although a Congressional funding ban still prevents them from receiving US government support. Although evidence indicates that syringe exchange is effective in slowing the spread of HIV in IDU (see, for example [6–8]) socially conservative American politicians have embraced contrary evidence from two Canadian cities [9,10] to assert that syringe exchanges are not only immoral because they encourage drug use, but may also actively spread HIV [2]. The purpose of this review is to provide a comprehensive, critical review of published evidence of the effectiveness of syringe exchange programs in reducing HIV risk behavior and HIV seroconversion among IDU. While five previous reviews have been published, four [5–8] are now seriously out of date, and a fifth [11] did not attempt a comprehensive critical review of the published literature. We have identified and will describe and discuss 42 studies published between 1989 and the end of 1999 that evaluated syringe exchange effectiveness. Identification of studies We systematically searched the published scientific literature in the English language using Medline (National Library of Medicine, Bethesda, Maryland, USA) and PsychInfo (American Psychological Association, Washington DC, USA) data bases for the period 1989–1999 using the key words 'needle exchange' and 'syringe exchange'. We then supplemented these searches by review of bibliographies in papers that came to our attention as well as inquiries among colleagues. We did not attempt to review unpublished reports, but we did examine published commentaries on research findings. Our searches did not uncover any published work on syringe exchange effectiveness in languages other than English. Almost all of the studies we found were conducted in the USA, Canada, the UK, or the Netherlands. We included all published studies of the effectiveness of syringe exchange programs (SEP) in reducing HIV risk behavior and HIV seroconversion, regardless of design. We also included the few studies we were able to find that examined effects of syringe exchange on hepatitis B (HBV) and hepatitis C (HCV) seroconversion, as hepatitis and HIV are transmitted in very similar ways and are, therefore, good biological markers for behaviors that place drug users at risk for HIV (they are also obviously important biological outcomes in their own right). The authors each independently reviewed each of the studies, and abstracted essential information concerning design, procedures, results, statistical controls, and study limitations. We then reconciled our ratings of the studies, arriving at a consensus description of each. We did not attempt a formal meta-analysis because outcome measures and/or data analysis differed markedly across studies. Criteria used in appraising the strengths and limitations of the studies included the adequacy of statistical controls, including adjustment for selection bias; the sensitivity of outcome measures used, statistical power, and where applicable, the meaningfulness of contrasts between users and non-users of SEP. In longitudinal studies, potential limitations include biases related to loss of respondents to follow-up, and in the case–control studies, failure to properly match cases and controls. In the case–control studies, recall bias is another potential limitation. Still another potential problem in longitudinal studies of syringe exchange is low baseline HIV seroprevalence and the related issue of restriction of person-time experience of incidence. Of the 42 studies reviewed, 23 were community studies in which the behavior and HIV status of users of SEP were compared with those of IDU not using SEP. Another 11 studies were conducted exclusively with SEP clients. Two studies were conducted with both community samples and with SEP clients. In addition, there were six studies that evaluated the ecological impact of SEP. The 42 studies used a variety of designs, including longitudinal or prospective cohort (10 studies), case–control (five studies), and multiple cross-sectional or observational (five studies and 16 studies, respectively). Five studies were based on evidence gathered at a single syringe exchange and used to model mathematically syringe exchange effectiveness. In describing the 42 studies, we group them first by study type (community study, syringe exchange study, ecological study), and second, within type by design (longitudinal/prospective, case–control, observational, modeling). The reader will note that several studies [9,12,13] used more than one design; for the sake of simplicity, we will discuss results obtained with each design separately. Community studies with longitudinal/prospective design Seven studies followed samples of IDU longitudinally, and compared HIV status and behavior of drug users who did versus those who did not use or attend syringe exchange (see Table 1). We will use the term 'use' and 'attend' interchangeably to mean that IDU themselves visited a syringe exchange for the purpose of exchanging.Table 1: Longitudinal/prospective cohort studies conducted with injecting drug user (IDU) community samples. In a study conducted in Montreal, Canada, Bruneau et al.[9] followed a cohort of respondents for a mean of 21 months, finding any SEP use in the previous 6 months to be a risk factor for seroconversion [adjusted odds ratio (OR), 1.7); the study differed from the majority of the studies described here in that IDU clients were predominantly cocaine users and had legal access to sterile syringes through pharmacies. In a recently presented abstract [15], however, Bruneau's team found no relationship between SEP use and HIV seroconversion, suggesting that the association may fade or be unstable over time. Des Jarlais et al.[16] performed a meta-analysis of three studies of SEP clients, vaccine preparedness respondents, and respondents in high-risk National AIDS Demonstration and Research cities, all of which were conducted in the USA. Controlling for potential confounders identified in a previous study, the investigators found that not using SEP continuously to be risk factor for HIV seroconversion (adjusted OR, 3.5). By contrast, Hagan et al.[17] in Seattle found no protective effect against HBV- and HCV-seroconversion of former, sporadic, or regular SEP use; their analysis adjusted for recent initiation of injection drug use and daily injection at baseline, both of which appeared to confound the effects of SEP use. In Portland, another West Coast US city, Oliver et al.[14] followed peer outreach and SEP clients over 6 months; although there was risk reduction in both groups, SEP clients were more likely to stop reusing 'works' without cleaning them, and were also less likely to throw away syringes (instead of returning them to then SEP); the investigators, however, did not adjust for preexisting differences between the two groups. In Vancouver, Canada, Schechter et al.[18] followed 694 treated and untreated IDU over 15 months, finding that HIV incidence was higher in frequent versus infrequent SEP attendees; however, when a lengthy list of potential confounders were controlled for, no relationship was observed between incidence and SEP use. Of added interest, the data showed that only 0.7% of the sample reported meeting new friends or other people there, suggesting that SEP use does not contribute to new sharing relationships. In New York City, Schoenbaum et al.[19] followed 329 IDU in methadone treatment over a period of 8 years. Following the introduction of SEP, they found increasing behavioral differences between SEP users and non-users, with the SEP users significantly less likely to report sharing syringes 4 years following introduction of the SEP. Interestingly, data collected from these methadone clients before SEP was implemented showed that there was less sharing of injection equipment among methadone patients who would later become SEP clients, suggesting that some of the later differences may have been due to client self-selection. van Haastrect et al.[20] also followed IDU over 8 years, examining predictors of mortality in HIV-positive and HIV-negative drug users in Amsterdam; participation in syringe exchange was not associated with a lower mortality rate. It is important to note that IDU in Amsterdam could legally purchase pharmacy syringes in addition to obtaining them from a syringe exchange. Community studies using multiple cross-sectional designs We identified four multiple cross-sectional studies (see Table 2). Bluthenthal et al.[12] interviewed untreated IDU in six waves of interviews conducted over 3 years in the San Francisco Bay Area. With each successive wave, the proportion of respondents reporting using a SEP to exchange syringes and obtain supplies increased, and there were at the same time decreases in the proportion reporting sharing syringes and supplies. Broadhead et al.[21] studied the impact of the closure of a Windham (USA) syringe exchange; rates of reusing of syringes increased from an average of 3.5 times monthly before or during closure to 7.7 times after; self-reported sharing of syringes also increased from 16% to 34%. Des Jarlais et al.[22] in New York City used a design very similar to that of Blunthenthal et al., except that the cross-sectional surveys were conducted quarterly over 3 years. Quarterly level of use of syringe exchange use correlated -0.67 and -0.44, respectively, with the proportion of respondents reporting borrowing and lending of syringes. In the fourth study, conducted by Singer et al.[23] in Hartford, successive decreases in reuse of syringes and supplies followed legalization of pharmacy sale of syringes, and later, the introduction of an SEP.Table 2: Multiple cross-sectional studies conducted with injecting drug user (IDU) community samples. Case control studies with community samples We identified four case–control studies which evaluated syringe exchange effectiveness (see Table 3). In a case–control study embedded in the longitudinal study already described, Bruneau et al.[9] compared 88 HIV seroconverters with a matched sample of 320 seronegative IDU. Surprisingly, cases were more likely to use syringe exchange [adjusted odds ratio (AOR), 4.2], when a host of other variables were controlled. As noted, this was a largely cocaine-using sample obtained in a city where pharmacy sale of syringes is legal. A very different result was obtained by Hagan et al.[13,24] in Tacoma USA. Matching cases and controls on demographic characteristics and injection frequency, they found 'never use' of SEP to be associated with a six- and sevenfold increased risk, respectively, of acquiring HBV and HCV. In Vancouver, Canada, Patrick et al.[25] conducted a case–control study to identify predictors of HIV seroconversion. No association was found between ever use of SEP and HIV seropositivity; 98% of the cases and 89% of the controls used an SEP. It is also worth noting that syringes may be purchased legally in pharmacies in Vancouver. Finally, van Ameijden et al.[26] in Amsterdam found no protective effect related to the proportion of syringes exchanged at an SEP; in Amsterdam as in Vancouver, IDU can obtain syringes legally from pharmacies.Table 3: Case–control studies conducted with injecting drug user (IDU) community samples. Observational studies conducted with IDU community samples Thirteen observational studies of IDU were identified, the bulk of which compared users and non-users of syringe exchange; several studies, however, compared IDU according to their frequency of SEP use (see Table 4).Table 4: Observational studies conducted with injecting drug user (IDU) community samples. Bluthenthal et al.[12] found a > 40% protective effect (AOR, 0.57) associated with SEP use, controlling for previously identified confounders. Bruneau et al.[9], obtained very different results in Montreal, finding an AOR of 2.2 for respondents reporting recent use of the SEP. As noted, this was a study conducted primarily with cocaine injectors in a setting where syringes could be purchased legally. In London, where pharmacy purchase of syringes was also an option, Donoghoe et al.[27] found users and non-users of SEP did not differ with regard to the extent to which they shared used injection equipment. The same was not true of a sample studied by Frischer et al. in Glasgow [28,29]; there, SEP attenders were less likely (mean 4.5 times/month) than non-attenders (mean 9.2 times/month) to borrow used syringes, despite legal pharmacy sale of syringes. Two studies conducted by Hartgers et al. in Amsterdam [30,31] found contradictory results. In the first study, exchangers were less likely than non-exchangers to have borrowed syringes during both the previous month (10% and 23% respectively) and the previous 2 years (33% and 57% respectively). In the second study, regular exchanging was not associated with increased or decreased borrowing. The second study controlled for previously identified confounders. As noted, IDU in Amsterdam could legally purchase syringes in pharmacies. The same was true of IDU in a study conducted by Keene et al.[32] in Wales. The investigators, however, found use of the SEP to be strongly protective against borrowing of syringes; only 9% and 10% of attenders reported borrowing in 2 consecutive years, compared with 41% and 39% of non-attenders. The investigators did not control for potentially confounding variables. Klee et al. conducted several studies of SEP in the northwest of England [33,34]. In these studies, SEP use was not consistently associated with accepting used syringes; in one, use of SEP was associated with passing used syringes to others. In Vancouver, Canada, Strathdee et al.[10] found frequent attenders (more than once a week) of the SEP were somewhat more likely (81%) than non-attenders (71%) to be HIV positive; a wide variety of potential confounders was controlled, and the AOR for HIV seropositivity of SEP use was small but significant (AOR, 1.68). However, a later study already referred to [18], which was conducted in the same setting, found that when additional confounders were controlled for, no relationship was observed between incidence and SEP use. In Amsterdam, a study conducted by van Ameijden et al.[35] found bivariate negative associations between SEP use and borrowing and lending of syringes; these associations, however, did not survive multivariate analyses. In a later study, also conducted in Amsterdam, van Ameijden and Coutinho [36] examined the impact of regular versus irregular versus no use of an SEP, adjusting for a number of possible confounders. Irregular use of the syringe exchange was associated with an 11.6% excess risk compared with regular users, but non-users were at 2.9% decreased risk, suggesting that non-users had other stable sources of syringes, possibly through pharmacies. In San Francisco, Watters et al.[37] found an almost twofold protective effect of SEP use (AOR, 0.54) with regard to reported needle sharing during the previous 30 days. Longitudinal/prospective cohort studies conducted with SEP clients Four studies followed SEP clients prospectively, comparing behavior reported at the point clients began to use syringe exchange with that at a follow-up interval (see Table 5). Of the four, only Donoghoe et al.[38] had a non-SEP attending comparison group. Among SEP attenders, 20% who reported sharing at baseline had ceased sharing by a 2 month follow-up, although 4% who reported no sharing at baseline reported it at follow-up; no changes in sharing were observed in the comparison group. In a study conducted by Hagen et al.[13] in Tacoma USA, the portion of SEP clients who reported borrowing and lending of syringes in the previous month declined from 58% and 72%, respectively, at baseline to 33% and 46% at follow-up. In nearby Portland USA, Oliver et al.[14] found that the proportion of clients who reported borrowing a syringe declined from 20% at initiation of SEP use to 7% at a 6 month follow-up, and the proportion of clients who reported renting a syringe decreased from 9% to 3%. Using a similar follow-up interval, Vlahov et al.[39] in Baltimore USA, found a 22% to 8% decrease in borrowing a used syringe, a 27% to 12% reduction in lending a used syringe, and additional reductions in indirect sharing (use of 'cottons' or 'cookers,' or using a syringe to divide drugs).Table 5: Longitudinal/prospective cohort studies conducted with syringe exchange program (SEP) clients. Observational studies conducted with SEP clients We located four observational (one-shot) studies conducted with SEP clients (see Table 6). In two studies from San Francisco USA [40,41], Guydish et al. found that recent users of syringe exchange had sharing in the first study, number of sharing was correlated with number of exchange in the 30 but was not correlated with number of sharing In the second study, a finding of decreased of was only a but respondents who did not report sharing obtained a somewhat higher proportion of their syringes from SEP versus et in New York City, interviewed IDU concerning their behavior during the previous 30 and the 30 the initiation of SEP use. The proportion who reported borrowing used syringes decreased from at initiation of SEP use to 12% at follow-up, and renting or used syringe decreased from 22% to Observational studies conducted with exchange (SEP) clients. Multiple cross-sectional study with SEP clients et syringe exchange over 4 years, but did not attempt to the to of study can be properly as multiple 4 years, the investigators found that the number of times shared decreased from to and the number of people they shared with decreased from two to one (see Table Multiple cross-sectional study conducted with exchange (SEP) clients. studies We identified five ecological studies of SEP effectiveness. Des Jarlais et described the characteristics of HIV in five cities among the five cities were of prevention when HIV seropositivity was still of sterile injection and et found that syringe sharing decreased of needle and drug there was also evidence that pharmacy sale of syringes increased following et found that seropositivity increased by in cities without SEP, and decreased by in cities with SEP. et found that seroprevalence of HBV and was not significantly different for IDU who began injecting before versus of an SEP, differences in of et found a seroprevalence of in a city in with an SEP, compared with to in other without that there is evidence that of syringe and of methadone and outreach programs an HIV from in all but a few studies and in New USA, conducted a of studies to model the effectiveness an SEP in reducing the time of syringes exchanged at the SEP. that with decreased time from increased the HIV infection in syringes would also decrease to be the 3 months of the syringe the time decreased from to and the of syringes that were decreased by from to also estimated the time before introduction of syringe exchange to be days. incidence among exchange clients was estimated first as new infections drug injectors with a interval of and later as new infections drug users with a of In another study they the decrease in proportion of to the SEP was not due to a in of clients Although the that these studies provide are they are to the same limitations of all studies, only an to the and that are shared among all not among small of also the of needle exchange and the of as well as the of syringes of all syringes in the there is evidence that syringe exchange programs are effective in HIV risk behavior and HIV seroconversion among IDU. Of the 42 studies reviewed found effects associated with use of syringe two found negative associations, and found no association or a of and negative The of the results differed according to the research design in The negative or were in the studies conducted with IDU community which compared users with non-users of syringe exchange. associations were in studies, negative associations in and effects in By contrast, all of the studies conducted with SEP clients out of found five of the six ecological studies effects associated with use of syringe exchange. The five studies by and conducted with syringes as to clients, also evidence of syringe exchange effectiveness. The negative and are both and to A by Strathdee et al.[10] for their own negative is that syringe exchange may not be to spread of HIV in IDU. The investigators studied they describe as the and syringe exchange in both in of the of syringes exchanged and the proportion of IDU despite access to sterile syringes, exchange clients behavior that themselves and at risk of and were more likely than to Of the seroconverters identified, only two reported access to syringes, reported no with regard to While we that syringe exchange may not be to spread of HIV among we that selection and biases may have to the negative and reviewed selection in a study that IDU in San Francisco over time from before syringe exchange was et found a seroconversion of among who would use syringe compared with among those who did Schoenbaum et al.[19] obtained similar findings. In their study, before of an SEP, IDU who would later use syringe exchange were more likely to report sharing syringes. Schechter et al.[18] that selection bias could for the higher among frequent as to infrequent attenders of SEP, and Hagan et found not only that new exchangers were at higher risk than but also that higher risk IDU were less likely to stop using an SEP over the of a and Bruneau to for a higher HIV seroconversion among exchange clients than in other studies have that cocaine users the bulk of the sample in the study are more likely than users to in high-risk behavior and also obtained evidence that to these findings. found that cocaine users more more in the of and a syringe to with the use syringes that more and attend more and more in work and/or with also that users in have a more stable and are more likely than cocaine users to purchase syringes in pharmacies during than attend the syringe exchange. In a recent published on and related to syringe exchange Vlahov not only on access to pharmacy syringes may differences between users and non-users of syringe but also three studies suggesting that IDU who obtain their syringes from pharmacies are more socially that those who attend syringe exchange; one study evidence of the to syringes or not based on the we that an additional factor the negative and was of other sources of syringes. this is were the contrasts between SEP clients and with access to other sources of syringes would not be very in could be an of where of IDU to obtain syringes. It is not a that the negative and were all from studies conducted in in those conducted in and the and the where IDU have access not only to syringe exchange but also to pharmacies where they can purchase syringes legally at low Five of the studies however, found effects of syringe exchange in with pharmacy that access is not a study limitation. among the IDU community studies a of legal pharmacy access by of versus negative and of SEP effectiveness this relationship to be significant at would to the recent of that the Canadian and Seattle studies a in the that needle exchanges provide an to the problem of transmission among drug the pharmacy access the outcomes of these studies, that may not be It be noted, however, that may not syringes to people they to be drug users it is legal to in Canada, the sale of syringes is to the of individual The that some SEP may higher risk IDU or that IDU may have access to other sources of syringes, however, at in not to of syringe exchange effectiveness. With longitudinal data differences between IDU who versus those who not to use syringe exchange selection could be controlled in the impact of syringe exchange In the of of syringe exchange have been an study would a prospective cohort and interviewed before of syringe that IDU who would later on to use an SEP could be in of risk from those behavioral data from a study would provide data for controlling confounding than was obtained in nearly all the studies reviewed of which used frequency of for risk behavior than behavior other sources of syringes to IDU is also critical to properly the impact of SEP. one of the 42 studies reviewed here did It is likely that failure to obtain measures of the extent of these other sources and/or use them in data analysis to failure to find differences between IDU who did versus IDU who did not use syringe exchange. IDU obtain the syringes from who attend syringe exchange for them is obviously another factor which been and to be and in SEP effectiveness. In it is worth several less with research on syringe exchange effectiveness. outcomes were used in only five studies, despite more less than self-reported which is to Interestingly, of the three studies using HBV or seroconversion as one found a decreased risk of seroconversion associated with use of SEP, the other found no despite the that HBV and are transmitted more than and are more markers of infection However, IDU not syringes but drug and drug as and it would be more to differences between users and non-users of an SEP additional of research on SEP to with adjustment for confounders. A of (see and a to controlling confounders. In not of the studies reviewed here observational, and case–control no attempt was to identify and control for confounders that for differences between IDU who did versus those who did not use syringe exchange. By we mean a review of literature on confounders identified in previous studies, and of these where they are in data In the of an of differences the could be due to or variables. Still another problem was the of SEP use. We have that use of SEP was not in any of these studies. In addition, however, measures as SEP once or more in the 6 were In our measures are not to the effects of SEP. In we that the of of SEP to be we are to to a of the true impact of syringe exchange.
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