Syringe exchange programs (SEPs) have been developed to reduce the spread of blood-borne pathogens such as HIV, and hepatitis B and C viruses (HBV, HCV) among injection drug users (IDUs). Studies have found that SEPs are effective at lowering and reducing injection-related risk behaviors [1–4], as well as reducing the incidence of HIV, HBV, and HCV among IDUs. [5,6]. Yet, in a recent study in Montreal, Canada, SEP users were found to have higher HIV incidence [7–9] and in studies in Vancouver, Canada and Seattle, Washington, SEP users received no protective effects against HIV, HBV, and HCV seroconversion [10,11]. These discrepant findings are significant and confound the emerging scientific consensus that SEPs should be a part of any comprehensive program to prevent the spread of HIV among IDUs [12–14].
Two factors may explain these discrepant results. First, in several studies, IDUs who self-selected into SEP use were found to have engaged in behaviors that placed them at greater risk for infectious disease seroconversion than non-SEP users. For instance, in Seattle, regular SEP users injected more frequently and reported more risk behaviors than sporadic and non-SEP users . In the Montreal and Vancouver studies, SEP users had higher baseline levels of risk behaviors and frequency of injection in comparison with sporadic or non-SEP users [7,15]. Adjusting for these baseline differences in post hoc statistical analysis may not provide accurate estimates. For instance, the Montreal study found HIV incidence among SEP users to be two-and-a-half times greater than the incidence in the comparison group. However, if baseline risk among the SEP users were accounted for, the HIV incidence in this group should have been five times greater than the comparison group, not merely two-and-a-half times greater . In Vancouver, subsequent analysis indicates that SEP use did not increase HIV seroconversion or HIV risk behaviors among IDUs .
Second, in all three discrepant cases, Montreal, Seattle, and Vancouver, IDUs had access to sterile syringes through over-the-counter sales at pharmacies. As Hagan et al. have remarked, this situation would tend to make SEP users and non-SEP users similar with respect to access to sterile syringes . An alternative interpretation of the discrepant results is that IDUs who have the means to purchase syringes through pharmacies are at lower risk for HIV, HBV, and HCV infection than their SEP-using counterparts. Possessing the means to purchase syringes at pharmacies may be an economic marker for other characteristics that may reduce HIV risk among non-SEP users. For instance, low income has been found to be associated with elevated risk of HIV/AIDS in a number of studies [18–20]. Other income-related risk factors such as injecting outdoors or in shooting galleries [21,22], pooling money to purchase drugs , and exchanging sex for money or drugs  may account for seroconversions among SEP users.
A significant problem for determining the impact of SEPs on HIV risk and incidence is accounting for possible baseline differences in the HIV prevalence and risk among IDUs under conditions where individuals are self-selecting into SEP use. One way to overcome this problem is to examine the impact of SEP use on a cohort of IDUs who self-reported syringe sharing and resided in a community where the only legal or semi-legal source of syringes was the SEP.
In this paper, we present data from a dynamic cohort of IDUs who reported sharing syringes within the 30 days prior to an initial interview (referred hereafter to as high-risk IDUs). IDUs in the study community could exchange syringes at one of the largest SEPs in the United States , but could not purchase syringes over-the-counter at pharmacies due to state syringe prescription laws . To measure the impact of SEP use on high-risk IDUs, we looked at factors associated with cessation of syringe sharing at a 6-month follow-up interview using logistic regression analysis.
Using targeted sampling methods [27,28] IDUs were recruited from two Oakland, California neighborhoods into a dynamic cohort study over 10 semi-annual samples (or `waves') from 1992 through 1996. Recruitment was conducted by project staff and through word of mouth in the IDU community in areas known for having a high concentration of drug users. Potential subjects were told about the purposes of the research, the location of community field sites, the hours of operation, and the eligibility criteria.
Eligibility criteria for initial participation were evidence of recent drug injection and being at least 18 years of age. To verify injection drug use, potential participants were asked to show `track' marks (visible scarring of skin along surface vein areas due to repeated injection) to study personnel at intake. Individuals without sufficient track marks were not enrolled into the study. IDUs enrolled in any wave were eligible and encouraged to return in subsequent waves, even if they stopped injecting drugs. Follow-up was accomplished by sending letters to respondents who provided a mailing address, through street outreach by study personnel, and by word of mouth among IDUs.
After obtaining informed consent, respondents were interviewed using the National Institute on Drug Abuse's Risk Behavior Assessment at initial interview, or the Risk Behavior Follow-up Assessment at subsequent interviews . An extensive local supplement was used at both initial and follow-up interviews. Respondents were questioned about their current and past drug use, sexual practices and preferences, medical history, knowledge and beliefs about HIV/AIDS, and utilization of HIV prevention services including syringe exchange. Following interview and HIV pre-test counseling, blood samples for HIV testing were drawn from respondents by trained phlebotomists. Enzyme immunoassay (EIA) was used to test blood samples for HIV antibodies. Samples repeatedly EIA-positive were confirmed using Western blot assay. Respondents then received an appointment to return to the field site within 4 weeks for post-test counseling and results notification. Respondents received a cash stipend for participating in the study.
In July 1992, an SEP opened in the study community. Due to limited funding, from July 1992 to October 1993, syringes exchanged per client were capped at 40 syringes per week. After October 1993, unlimited one-for-one exchange of syringes was provided. The SEP added additional sites in 1993 and 1994, and as of November 1999 is open for 6 h per week at three street-based sites in the City of Oakland. For IDUs presenting at SEP sites without syringes, 10 syringes were provided following determination of drug injection through visual inspection for track marks. SEP consumers also received condoms, clean cotton, and alcohol wipes. Trained and experienced outreach workers were available at each site to provide referrals and active assistance in obtaining medical and social services, including drug treatment, upon request.
Of the 1503 respondents in the overall study, 549 respondents had participated in at least two interviews within 6 months of each other. We selected the latest interview at which a respondent reported syringe sharing and for which we had a follow-up interview within 6 months to maximize our ability to examine the impact of SEP use on high-risk IDUs. (The proportion of IDUs in the study reporting SEP use increased from less than 5% in 1992 to nearly 60% in 1996). Three hundred and forty respondents met this criteria and were selected into our high-risk cohort. As compared to single-visit high-risk respondents, high-risk cohort members were significantly more likely to be over 40 years of age (69 versus 50% for single visit;P < 0.001), to be housed (73 versus 60%;P = 0.003), to be currently in drug treatment (15 versus 5%;P < 0.001), to have a IDU steady sex partner (48 versus 39%;P = 0.036), and to have medical insurance (54 versus 43%;P = 0.009). In comparison with high-risk IDUs who were seen more than once, but not within a 6-month follow-up period, cohort members were less likely have smoked crack in the 30 days prior to initial interview (59 versus 76% for multi-visit non-cohort;P < 0.001).
Syringe sharing, our outcome variable, was defined as self-reported use of a syringe that had been used by somebody else in the 30 days prior to interview. SEP users were defined as IDUs who had used the SEP in the 30 days prior to interview and reported the SEP as their usual source of syringes in the past 6 months. To capture the dynamics of SEP use, we divided the high-risk cohort into four groups based on their SEP utilization at initial interview and follow-up. IDUs who did not use the SEP at either interview were compared with those who initiated SEP use by the follow-up interview, those who used the SEP at both interviews, and those who used the SEP at initial interview, but not at follow-up.
To determine if initiation of SEP use and other factors predicted cessation of syringe sharing at the follow-up interview, we conducted bivariate analysis using Mantel–Haenszel χ2 test. Variables found to be significant in bivariate analysis or which previous research has suggested are meaningful were entered into a logistic regression model predicting cessation of syringe sharing. Because some respondents had participated in the study a number of times before meeting our selection criteria, we controlled for number of prior visits in the multivariate model. We also controlled for wave of study entry, due to the increasing availability of SEP services during the study period. Other variables considered in multivariate analysis included age, sex, race, homelessness, drug treatment, crack smoking, years of injection, and frequency of injection. SAS Statistical Software, Version 6.12 (Cary, North Carolina, USA) was used to perform all statistical analyses.
The demographic characteristics of the high-risk cohort (n = 340) at follow-up are reported in Table 1. The majority of the sample was African-American, male, and forty years of age or greater. Twenty-seven percent of respondents considered themselves to be homeless and 8% tested positive for HIV based on HIV testing conducted by the investigators. The median age of the sample was 43.1 years (inter-quartile range, 38.6–47.0).
In terms of drug most frequently used, 72% of the sample injected heroin, followed by 14% who smoked crack most often, 8% who injected speedballs (mixture of heroin and cocaine), and 2% who injected cocaine. The median duration of injection drug use career was 24.0 years (inter-quartile range, 16.5–28.3). The median times injected in the 30 days prior to baseline interview was 64.0 times (inter-quartile range, 30.0–96.0). Thirty day injection frequency declined substantially from baseline to follow-up interview for both IDUs who ceased sharing syringes (baseline median injections = 77.0 versus 63.0 at follow-up) and those who did not (84.0 versus 73.5). These declines in injection frequency were not associated with quitting syringe sharing in bivariate analysis (P = 0.69). Other variables not statistically significant in bivariate analysis include drug treatment entry in the 6 months prior to follow-up interview (P = 0.97), current enrollment in drug treatment (data not shown), and length of drug injection career (data not shown).
Overall, 60% (204 of 340) of respondents reported quitting syringe sharing at follow-up interview. Nonetheless, initiation and continuation of SEP use were associated with quitting syringe sharing. Among IDUs who began using the SEP at follow-up interview, 74.5% (41 of 55) reported not sharing syringes in the 30 days prior to follow-up interview (Table 1). For those who used the SEP at both interviews, 68.3% (43 of 63) reported cessation of syringe sharing. Among IDUs who did not use the SEP at either interview, 53.7% (101 of 188) quit sharing syringes at follow-up interview. For IDUs who used the SEP at initial interview but not at follow-up interview, the rate of quitting syringe sharing was 54.5% (18 of 33).
A third variable was significantly associated with quitting syringe sharing in bivariate analysis. IDUs who had a steady sex partner who was a drug injector were less likely to quit sharing syringes (52.1 versus 67.2% those without a IDU steady sex partner). Two other variables, smoked crack in the 30 days prior to follow-up interview and age less than 40 years, were nearly associated with quitting syringe sharing (P = 0.08).
To determine whether SEP use was independently associated with cessation of syringe sharing, we conducted multivariate analysis (Table 2). In analysis controlling for total number of prior interviews in the study, wave of study entry, age, IDU steady sex partner, and crack smoking in the 30 days prior to follow-up interview, IDUs who started SEP use [adjusted odds ratio (AOR), 2.68; 95% confidence interval (CI), 1.35–5.33;P = 0.005] and IDUs who continued SEP use (AOR, 1.98; 95% CI, 1.05–3.75;P = 0.03) were independently associated with quitting syringe sharing. Stopping SEP use was not significantly associated with quitting syringe sharing. Other models were tested that included frequency of injection, length of injection career, and sex (not shown). The inclusion of these variables did not change the association between SEP use and quitting syringe sharing.
In the context of overall declines in syringe sharing (60% reported cessation of syringe sharing) and no other legal or semi-legal access to sterile syringes, we found that IDUs who initiated and those who continued using the SEP were significantly more likely to quit syringe sharing than IDUs who did not use the SEP. These results are consistent with other cohort analyses of the impact of SEP use on IDUs in the United States [2,3,30] and confirm results of cross-sectional analyses among IDUs in the San Francisco Bay Area [1,4] which have found SEPs to be associated with lower injection-related risk among IDUs. Further, by comparing high-risk SEP users with high-risk non-SEP users, we have shown that SEP use contributed to reductions in HIV risk behavior among high-risk IDUs.
Although we did not examine social network formation among IDUs, our data provide some insight into speculation that SEPs serve as sites for the formation of high-risk social networks [7,10]. The greater than two-fold increase in the likelihood of quitting syringe sharing for initiates of SEPs suggests that even if social network formation were occurring at SEP sites it was likely to be occurring in the context of dramatic declines in syringe sharing. More recent reports indicate that IDUs are not likely to identify new drug injection partners through SEPs . Further, the finding that continuing SEP users were also more likely to stop syringe sharing points to the positive HIV prevention impact of SEPs on IDUs who choose to use them.
We also found that having a steady sex partner who is an IDU was associated with continued syringe sharing in our cohort. Others have found sexual/intimate partnerships among IDUs to be associated with syringe sharing [31,32]. The injection-related risks taken between IDUs in mutually dependent, sexual relationships has received little attention to date as a focus for risk reduction interventions . It is very difficult to change risk taking within the context of sexual partnerships  and SEPs do not specifically intervene on the steady sexual partnerships between IDUs. On the other hand, other studies have found that IDUs do distribute sterile syringes to their social network contacts, suggesting that far from being a source of more HIV risk [35–37], SEPs appear to be a source of HIV prevention for personal and social contacts of SEP users. Novel approaches to reducing both injection and sex risk among IDUs with sexual partners who are also drug injectors are needed.
We also observed a trend toward younger IDUs being less likely to quit syringe sharing. Initiation of drug injection typically involves sharing of injection equipment . Analysis of drug injectors recruited from drug treatment centers in San Francisco found that younger IDUs were more likely to share syringes . In an analysis of a SEP cohort in New York, IDUs with fewer years of drug injection were more likely to continue sharing syringes . Further, studies in various cities have found more recent initiates to drug injection and younger IDUs more likely to seroconvert to HIV and HCV [41–43]. Efforts to reach younger drug injectors and more recent initiates to drug injection are urgently required to stem the spread of HIV and other blood-borne pathogens among IDUs .
These findings should be viewed in the context of the following limitations. Due to the clandestine nature of drug use, no sampling technique can randomly select research participants. Thus, our findings may not apply to other drug injectors in Oakland and elsewhere. Further, because refusal to participate in the study occurs outside the research venue, deriving a true refusal rate is impossible. Another concern arises from biases associated with using self-report data regarding sensitive and stigmatized behavior (i.e. drug use and HIV risk behaviors). The specific measures used in this study have been found to be reliable and valid among IDUs recruited through street outreach in other research settings [45,46].
Generalizing our findings to other participants in our study is another concern. To look at this, we compared IDUs in the high-risk cohort with single-visit IDUs who reported syringe sharing. In comparing cohort members with single-visit high-risk IDUs, cohort members were significantly more likely to be older, to be housed, to have been in drug treatment in the 30 days prior to initial interview, to have a IDU steady sex partner, and to have health insurance. High-risk cohort members were less likely to smoke crack in the 30 days prior to interview than other multi-visit respondents.
Another limitation may arise from the comparison group. The most straightforward approach, random assignment of subjects to SEP use and non-use, is widely considered unethical in locales where syringes are not otherwise available [14,47,48]. In cities where syringes are available through over-the-counter pharmacy sales, comparisons between SEP users and non-users may be less relevant [48,49]. Other potential comparison groups include drug treatment cohorts or IDU cohorts in communities without SEPs. Drug treatment cohorts are inappropriate since drug treatment is an effective HIV prevention intervention in its own right . Comparison between communities is problematic because of the wide variety of factors that can influence injection-related risk behaviors at the community level such as law enforcement practices, availability of drug treatment, socio-demographic and economic differences, and the availability and form (i.e. powder versus tar) of illicit substances. We have attempted to overcome these various problems by having a comparison group of IDUs at acknowledged high risk that reside in a community with syringe access limited to the SEP.
A recent literature review concluded that of the 35 studies published prior to 1998, 27 reported that SEPs reduced injection-related HIV risk behaviors, six showed mixed or neutral results, and two found negative impacts [51,52]. This study can be added to the growing list of studies which have found SEPs to be effective at reducing HIV risk behaviors among IDUs. Despite the political controversy that surrounds SEPs, the fact is that they are among the most effective HIV prevention programs for IDUs and should be implemented in communities that have been heavily impacted by injection drug use.
This study was supported by grants from the Community Research Branch of the National Institute on Drug Abuse (grants RO1-DA09532 and UO1-DA06908). Human subjects protocols were approved by the institutional review board at the University of California, San Francisco. All subjects provided written consent to participate in the study.
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