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Representativeness of Injecting Drug Users Who Participate in HIV Surveillance: Results From Australia's Needle and Syringe Program Survey

Topp, Libby PhD*; Iversen, Jenny BAppSci*; Wand, Handan PhD†; Day, Carolyn PhD‡; Kaldor, John PhD§; Maher, Lisa PhD*; on behalf of the Collaboration of Australian Needle Syringe Programs

JAIDS Journal of Acquired Immune Deficiency Syndromes: 15 April 2008 - Volume 47 - Issue 5 - pp 632-638
doi: 10.1097/QAI.0b013e31816a1d68
Epidemiology and Social Science

Objective: Australia's annual Needle and Syringe Program (NSP) Survey forms the basis of HIV surveillance among injecting drug users (IDUs) by providing serial point prevalence estimates of patterns of infection and risk behaviors. This study examined the representativeness of NSP Survey samples.

Design: National cross-sectional survey of 3920 NSP clients.

Methods: Demographic and drug use characteristics of respondents and nonrespondents to the 2006 Survey were compared.

Results: Relative to NSP clients who last injected heroin, methamphetamine injectors were significantly more likely to complete the Survey, as were people who had last injected an opioid maintenance pharmacotherapy or morphine. Other variables independently associated with Survey completion were female gender and being aged ≥35 years. Although the median age of NSP Survey samples has increased by 1 year per annum since 2002, the increase has occurred among both repeat and first-time respondents, allaying concerns that an ageing cohort of NSP clients repeatedly completes the Survey and unduly influences its results.

Conclusions: Inferences derived from the Survey results can reasonably be applied to the population of NSP clients, although because older female pharmaceutic injectors may be overrepresented among NSP Survey participants, recruitment strategies to target specific subpopulations (younger male participants) and stratification of main outcomes by age and gender in future analyses may usefully be considered. Although the extent to which Survey results can be generalized to Australia's broader IDU population cannot be ascertained, their consistency with other sources of surveillance data suggests that NSP Survey samples reflect the changing characteristics of Australia's illicit drug markets. Consequently, these are likely to be as representative samples of injectors as it is practical to obtain, and the Australian NSP Survey provides a useful model for blood-borne virus surveillance among IDUs.

From the *Viral Hepatitis Epidemiology and Prevention Program, National Centre in HIV Epidemiology and Clinical Research, University of New South Wales, Darlinghurst, New South Wales, Australia; †Biostatistics and Databases Program, National Centre in HIV Epidemiology and Clinical Research, University of New South Wales, Darlinghurst, New South Wales, Australia; ‡Drug Health Services, Central Clinical School (C39) Faculty of Medicine, University of Sydney New South Wales, Australia; and the §National Centre in HIV Epidemiology and Clinical Research, University of New South Wales, Darlinghurst, New South Wales, Australia.

Received for publication September 25, 2007; accepted January 22, 2008.

C. Day is supported by a National Health and Medical Research Council (NHMRC) Post Doctoral Fellowship, and L. Maher is supported by an NHMRC Career Development Award. The National Centre in HIV Epidemiology and Clinical Research is funded by the Australian Government Department of Health and Ageing and is affiliated with the Faculty of Medicine, University of New South Wales.

Correspondence to: Libby Topp, PhD, Centre for Health Research in Criminal Justice, Suite 302, Level 2, Westfield Office Tower, 152 Bunnerong Road, Pagewood NSW 2035, Australia (e-mail: libby.topp@justicehealth.nsw.gov.au).

Principal approaches to surveillance of infectious diseases such as HIV include (1) case reporting, (2) sentinel surveillance based on routine voluntary testing, and (3) generation of data among targeted high-risk populations.1 HIV surveillance among injecting drug users (IDUs) is thus an essential component of comprehensive HIV surveillance.2 Given the “partially hidden” nature of IDUs,3 and the consequent inability to construct sampling frames for this population,4 a range of surveillance programs have been implemented among defined subpopulations, including prospective cohort studies5 and serial cross-sectional surveys of IDUs recruited from community-based6 and/or treatment7 settings.

In Australia, where the low prevalence of HIV infection among IDUs renders prospective cohort studies prohibitively expensive for surveillance purposes,8 cross-sectional seroprevalence surveys of IDUs form the basis of HIV and hepatitis C virus (HCV) surveillance among this population. Since 1995, the Australian Needle and Syringe Program (NSP) Survey has provided annual estimations of point prevalence to monitor changes over time in patterns of infection and risk behaviors among NSP clients.9,10 During a 1-week period in October, staff at selected NSP sites in all Australian jurisdictions recruit clients to (1) complete a brief questionnaire covering demographics, injecting drug use, and blood-borne virus (BBV) transmission risk behaviors and (2) provide a capillary blood sample, which is subsequently screened for the presence of antibodies to HIV and HCV. Participation is voluntary and is not reimbursed. Sample sizes for the Survey have ranged from 1072 in 1995 to 2694 in 2000.11 Serology results are used to estimate the prevalence of HIV and HCV infection among NSP clients and are often interpreted as a proxy for the prevalence of these infections among Australian IDUs,12-17 including by international investigators.4

Thus, as is the case in most studies of illicit drug users, results from the NSP Survey are used to draw inferences about an entire population-in this case, NSP clients and, often, Australian IDUs. NSP Survey participants are by necessity, however, a convenience sample of NSP clients rather than being recruited through the more desirable probability sampling, in which each sampling unit has a known probability of being selected so that inferences about the population can be derived from the sample with a measurable degree of precision.18 Unlike the North American literature, which clearly indicates that NSPs attract marginalized IDUs at high risk of acquiring BBVs,19-21 the limited Australian research suggests that NSP attendees and nonattendees are demographically comparable, and similar proportions report recent sharing of needles and syringes.17,22,23 This lack of substantive difference may be attributable to the availability of NSPs in Australia, where they are legal and government funded and enjoy bipartisan political support as a fundamental BBV prevention strategy.24 These characteristics may contribute to the acceptability of their clients as a proxy population for IDUs in general. Nonetheless, the degree of external validity of results of the NSP Survey is difficult to determine.

This issue has acquired added significance in light of changes in Australia's illicit drug markets after the marked reduction in heroin availability in 2001. Whereas other research has demonstrated substantial changes in patterns of drug injection, with an overall shift from heroin to methamphetamine,25 notably among younger IDUs,26 between 2002 and 2005, heroin remained the drug most frequently reported as “last injected” among NSP Survey respondents.11 Consequently, an examination of the generalizability of the Survey's findings may be particularly timely. Moreover, the median age of NSP Survey samples increased between 2002 and 2005 by approximately 1 year per annum,11 raising concerns that an ageing cohort of IDUs who are engaged with NSP services may repeatedly complete the Survey and unduly influence its results. Understanding the way in which these issues affect the representativeness of Survey samples has broader implications for the utility of NSPs as surveillance sites, and more broadly, for the use of serial point prevalence studies as key surveillance methodologies. Should the NSP Survey manage only to attract older entrenched IDUs routinely, reservations about its value in surveillance would clearly be warranted.

Accordingly, this study aimed to:

1. Determine the representativeness of NSP Survey participants relative to the broader population of NSP clients by comparing basic demographic (age, gender) and drug use (self-reported drug last injected) characteristics of NSP clients who agreed to undertake the 2006 Survey (respondents) with those of NSP clients who declined to participate (nonrespondents). Specifically, we sought to examine the possibility that, particularly in the wake of changes to Australia's illicit drug markets after the 2001 heroin shortage, the NSP Survey preferentially recruits older heroin injectors relative to younger methamphetamine injectors.

2. Examine the possibility that an ageing cohort of repeat participants might account for the increase over time in the age of the samples of respondents.

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METHODS

Measures

Response Sheets

As part of the NSP Survey, staff members at participating NSP sites are asked to complete Response Sheets, which record information on every client attending the NSP during Survey week (ie, respondents, nonrespondents). These data are used to calculate national and jurisdictional response rates. In 2006, as in previous years, Response Sheets were recorded manually by NSP staff based on the verbal reports of all NSP clients, comprising (1) age, (2) gender, (3) whether they were new (“first visit”) or return clients for the week, and (4) whether they completed the Survey. Also recorded for the first time in 2006 was drug last injected. Most Australian NSPs record, by means of routine service level data collection systems, the age (most as a continuous variable but some categorically) and gender of their clients, and some also collect “last drug injected.” Thus, clients and staff are familiar and comfortable with the provision and collection of limited demographic information before clients collect their supplies; however, it is nonetheless imperative to minimize the burden on both groups of Survey participation. The extra variables added to existing data collections for the purposes of the Survey Response Sheets are thus first visit/return client; Survey participation status; and, for services that do not routinely collect it, last drug injected (recorded as an open-ended variable). In the limited instances in which clients refused to report this information, staff members were still able to record estimated age, gender, and Survey participation status.

Thirty-nine of the 45 participating sites completed Response Sheets (Fig. 1). Of these 39, data provided by 35 sites were considered sufficiently reliable for inclusion in further analyses. Response Sheets were deemed reliable when they (1) recorded a minimum of 10 client visits per day (because extremely low-volume NSP services do not participate in the Survey), (2) recorded Survey participation rates of <50% (in line with our consistent finding over 13 Survey years that one third to one half of clients agree to participate11), and (3) were completed on every day on which the Survey was undertaken. If Response Sheets are completed correctly, the key characteristics of those recorded on Response Sheets as having participated in the Survey should closely match the NSP Survey sample itself, which was indeed the case (data not shown).

At 3 of the 4 sites in which Response Sheets were deemed insufficiently reliable for inclusion in further analyses, most (>90%) of clients for whom a Response Sheet was completed were recorded as having completed the Survey at first visit. We subsequently confirmed with staff of these 3 services that they had mistakenly completed Response Sheets primarily for clients who agreed to participate in the NSP Survey rather than for all clients who accessed the service during Survey week. At the fourth site, Response Sheets were not completed for every day on which the Survey was undertaken, and thus could not represent every occasion of service to NSP clients during Survey week. Accordingly, we did not consider it methodologically sound to include data provided by these 4 sites in our analyses of the representativeness of NSP Survey participants.

Thus, a total of 10 NSP Survey sites did not contribute data to the present analyses because Response Sheets were not completed or their Response Sheets were deemed unreliable (see Fig. 1). These 10 sites represent a range of high-volume and lower volume NSP services located in urban, regional, and rural areas in 6 of Australia's 8 jurisdictions, and they are sufficiently varied that we cannot envisage any systematic bias in our present results attributable to their lack of inclusion in these analyses.

There were also instances in which sites operated at times when completion of the Survey was not possible (eg, because of limited NSP staff after hours). Clients attending these NSPs at such times were not invited to participate, and Response Sheets were thus not completed for these clients.

Although the Response Sheets collected data on first visit and return clients, identifying data were not recorded; therefore, it was impossible to match a return client's data to data collected at that client's first visit. To remove duplicate clients from further analyses, the analysis of response rate data was confined to records of a client's first visit during Survey week.

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NSP Survey Repeat Participants

The NSP Survey asks participants whether they have participated in the survey in previous years. Responses to this item were used to calculate the proportion of repeat participants in the NSP Survey between 2002 and 2006. The median age of repeat participants was compared with that of new participants (ie, those who reported that they had not previously participated in the Survey).

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Data Analysis

Data were analyzed with STATA 8.2 statistical software (Stata Corporation, College Station, TX). We modeled univariate and multivariate associations between demographic and drug use characteristics and completion of the Survey at first visit to an NSP site during Survey week. Wald χ2 tests and related 95% confidence intervals (CIs) were calculated to assess statistical significance and precision.

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Ethical Approval

Each year, the NSP Survey is subject to ethical approval from the broad range of relevant institutional, jurisdictional, and site-specific human research ethics committees.

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RESULTS

During Survey week in 2006, 5018 NSP client visits were recorded on Response Sheets. Of these, 1098 were excluded from these analyses because (1) the client was not invited to participate, (2) the client was a return client for the week, or (3) participation in the NSP Survey was not reported.

Of the 3920 client visits included in these analyses, 1415 clients agreed and 2505 declined to participate in the NSP Survey, equating to a response rate of 36%. Participation rates were highest among NSP clients who reported that the drug they had last injected was methadone/buprenorphine/buprenorphine-nalaxone combination (50%), followed by morphine injectors (44%), methamphetamine injectors (40%), heroin injectors (33%), and cocaine injectors (19%).

A number of demographic and drug use characteristics had significant univariate associations with NSP Survey participation at first visit (Table 1), including being female; being aged ≥35 years; and having last injected methamphetamine, methadone/buprenorphine/buprenorphine-nalaxone combination, or morphine rather than heroin. In contrast, IDUs who reported that they had last injected cocaine were significantly less likely to participate in the Survey at first NSP visit than heroin injectors.

These patterns of associations did not alter in multivariate analyses (see Table 1). Compared with men, women were more likely to participate in the Survey during first visit (relative risk [RR] = 1.4, 95% CI: 1.2 to 1.6). Older age was associated with increased likelihood of Survey participation (linear trend, P < 0.0001). Analysis of separate age categories indicated that although there was no significant difference in participation between NSP clients aged <25 years and those aged 25 to 34 years, older injectors (≥35 years of age) were 1.6 (95% CI: 1.3 to 1.9) times more likely to participate than those aged <25 years. Relative to heroin injectors, methamphetamine injectors were 1.5 (95% CI: 1.3 to 1.7) times more likely, methadone/buprenorphine/buprenorphine-nalaxone combination injectors were 2.2 (95% CI: 1.4 to 2.3) times more likely, and morphine injectors were 1.8 (95% CI: 1.3 to 1.9) times more likely to participate. Conversely, cocaine injectors were significantly less likely to participate than those who had last injected heroin (RR = 0.6, 95% CI: 0.4 to 0.9).

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Repeat Participants

Since 2002, the proportion of repeat participants in the NSP Survey has remained relatively low and stable, at between 18% and 23% (Table 2). Although the median age of repeat participants rose by 1 year per annum from 2002 to 2006, the median age of new participants increased in a similar manner over this period (Fig. 2), indicating that an ageing cohort of repeat participants is unlikely to account for the increasing age of NSP Survey samples.

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DISCUSSION

This study found that in the 2006 sample of NSP Survey respondents, older heroin users were not preferentially recruited relative to younger methamphetamine users. NSP clients who reported their last drug injected as methamphetamine were, in fact, more likely to complete the Survey at first NSP visit during Survey week than were heroin injectors. NSP clients who last injected methadone/buprenorphine/buprenorphine-nalaxone combination were most likely to participate in the Survey at first visit, followed by those who last injected morphine. Cocaine injectors were the least likely of all NSP clients to participate. Although these results cannot preclude the possibility that heroin users were preferentially recruited to the Survey in previous years, these data clearly indicate that this was not the case in 2006.

Although some NSP sites may have increased their efforts to recruit younger nonopiate users in 2006, this possibility is not consistent with findings that IDUs aged 35 years or older were more likely than their younger counterparts to participate. It is, of course, important that older IDUs are not overrepresented in surveillance research, because duration of injecting, which is highly correlated with chronologic age,10,27,28 is strongly associated with HCV seropositivity.29-32 Thus, HCV prevalence could be overestimated if older IDUs are preferentially recruited. Higher background prevalence may be associated with a “ceiling” effect, leading to difficulty in detecting trends over time in prevalence estimates. Conversely, a range of research, including prospective cohort studies of HCV incidence, suggest that new initiates to injecting may be particularly vulnerable to HCV infection and that the risk of seroconversion declines with age and duration of injecting.33-38

Compared with men, women were more likely to participate in the 2006 survey. This pattern was documented in the original NSP Survey implementation research8 and is likely to have been the case throughout the existence of the Survey. Research in other areas has also indicated that women are more likely to comply with requests to participate in health-related research,39 a pattern that may be explained by the generally more relational, expressive, and communal nature of women than men40 and the demands of conventional gender roles, in which women are perceived as more compliant and responsive to the needs of others than are men (compare articles by Crawford and Unger41 and Deutsch42). The potential that women may be preferentially recruited into surveillance studies suggests that recruitment strategies to target men, and stratification of main outcomes by gender in future analyses, may usefully be considered.

This study failed to support the hypothesis that the Australian NSP Survey continually accesses an ageing cohort of repeat participants who unduly influence the results. The proportion of repeat participants has remained relatively low and stable during the past 5 years, at approximately 20% of all Survey respondents. Although the median age of Australian NSP Survey participants has increased annually since 2002, the findings presented here indicate that this has occurred across all Australian NSP Survey participants rather than being confined to repeat participants, suggesting that the average age of NSP clients may have also increased over this period. This is consistent with other research, which has identified an increase in the median age of regular heroin users and a possible reduction in initiation to heroin injecting.43 It has been suggested that younger less entrenched heroin users may have dropped out of the heroin market altogether after Australia's 2001 heroin shortage.26 Indeed, since the heroin shortage, the size of the overall population of IDUs in Australia is estimated to have decreased.44,45

Increased use of drugs by means of noninjecting routes of administration (eg, smoking of crystalline methamphetamine46) may further explain this increase in the average age of Australian IDUs, which has also been detected by the Illicit Drug Reporting System (IDRS), Australia's strategic early warning system.47,48 This annual survey has also observed an increase in the average age of samples of IDUs recruited from a range of sites across Australia, from 30.1 years in 2002 to 34.1 years in 2005.49 Indeed, recent comparisons demonstrate striking similarities between the 2006 NSP Survey (N = 1961) and IDRS (N = 914) samples,50 with equivalent mean ages (34.8 vs. 34.5 years); proportions of men (65% vs. 64%), heterosexuals (82% vs. 86%), Indigenous Australians (10% vs. 13%), and participants with a history of incarceration (48% vs. 51%); daily or more frequent injection (47% vs. 46%); and an injecting career of 3 or more years (91% vs. 98%). Similar to the present study, Judd et al6 observed increasing median age among participants recruited for their serial cross-sectional point prevalence HIV surveys from community-based and treatment settings in Greater London but argued this was unlikely to be attributable to an ageing cohort because they observed a similar trend among opiate injectors presenting to treatment services in North London during the same period.

Consistent with international findings,7,51,52 our data do not support the proposition that the availability of NSP services entices new initiates to injecting or promotes a transition from noninjecting to injecting drug use. Since 1999, Australian NSPs have received increased funding to diversify and expand services,53 yet the data presented here, along with a range of other data sources from Australia's comprehensive illicit drug surveillance systems,45 suggest that in recent years in Australia, there has been a decrease in the proportion of IDUs aged ≤25 years. A reduction since the heroin shortage in mandatory notifications to jurisdictional health departments of incident HCV cases among 15- to 19-year-olds has been attributed to decreased initiation of injecting among this group.54

A limitation of this study, and the NSP Survey itself, is the lack of information that is collected from NSP clients who decline to participate, particularly in terms of their risk behaviors. Although the Survey is designed to place as few demands as possible on respondents, who are not reimbursed for their participation, it does involve providing a capillary blood sample and answering a series of questions relating to illicit and socially stigmatized activities; only approximately one third to one half of clients who attend NSPs during Survey week agree to participate.11 Nonetheless, the addition to Response Sheets (which collect information on all clients during Survey week, including nonrespondents) in 2006 of “drug last injected” was a significant improvement on the procedure in previous years, allowing a better delineation of population parameters and, consequently, a greater capacity to address issues around sample representativeness.

Notwithstanding some (inevitable) bias, large and broad cross-sectional samples of IDUs are recruited each year through this system. Our results thus indicate that, at least in the context of relatively widespread, legal, publicly funded NSPs, such as exist in the United Kingdom, Europe, and, more recently, Canada, such sites can facilitate indispensable surveillance. As demonstrated by our data and elsewhere,6 ageing cohorts need not be interpreted as artifacts of specific methodologies; serial cross-sectional studies can, in fact, detect such phenomena. This study also illustrates the need to consider the issue of gender closely in surveillance with IDUs. Although women are understood to be oversampled in health studies in general, the disproportion of women among IDUs adds to the complexity of assessing the representation of women in research with this population.

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CONCLUSIONS

These results demonstrate that the concern regarding the Australian NSP Survey preferentially recruiting heroin injectors relative to methamphetamine injectors was not warranted, at least in 2006, when, in fact, methamphetamine injectors were more likely than heroin injectors to complete the Survey. The results also indicate that most participants in the Survey are new to the survey each year and that repeat participants are unlikely to exert an undue influence on the Survey's results. The annual increase in age of NSP Survey participants is not attributable to the increasing age of repeat participants but likely reflects the increasing age of the broader NSP population and, when considered in the light of data drawn from other illicit drug surveillance systems, probably the population of IDUs as a whole. Our findings suggest that inferences derived from the results of these Surveys can reasonably be applied to the population of NSP clients, although because older female pharmaceutic injectors may be overrepresented among NSP Survey participants, recruitment strategies to target specific subpopulations (younger men) and stratification of main outcomes by age and gender in future analyses may usefully be considered. Nonetheless, although the extent to which NSP Survey results can be generalized to Australia's population of IDUs as a whole cannot be ascertained, given the “partially hidden” nature of illicit drug injectors,3 we conceive of the NSP Survey samples in the same manner in which Hope et al4 described their samples of IDUs recruited from drug treatment and nontreatment community settings: a “sample (that) is likely to be as representative a sample of injectors as it is practical to obtain.”

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APPENDIX

Collaboration of Australian Needle and Syringe Programs: Directions Australian Capital Territory (ACT); AIDS Council of NSW (Sydney and Hunter); Albury Base Hospital and Albury Community Health Center, Albury; First Step Program, Port Kembla; Health ConneXions, Harm Reduction Program, Liverpool; Hunter NSP Services, Royal Newcastle Hospital, Newcastle; Indo-Chinese Outreach Network (ICON), Bankstown, Cabramatta, and Liverpool; Kirketon Road Center and K2, Kings Cross; NSW Users and AIDS Association (NUAA), Surry Hills; Northern Rivers Area Health Service, Ballina, Byron Bay, Lismore, Murwillumbah, Nimbin, and Tweed Heads; Resource and Education Program for IDUs, Redfern and Canterbury; Responsive User Services in Health (RUSH), Manly, Ryde, and St. Leonard's; St. George NSP, Kogarah; Southcourt Primary Care NSP, Nepean; Sydney Sexual Health Center, Sydney; Sydney West Area Health Service HIV/Hepatitis C Prevention Service, Auburn, Blacktown, Merrylands, Mt. Druitt, and Parramatta; Northern Territory AIDS Council, Alice Springs, Darwin, and Palmerston; Biala Community Alcohol and Drug Services, Brisbane; Cairns Base Hospital NSP, Cairns; Cairns Youthlink, Cairns; Logan Youth Heath Services, Logan; Mackay Sexual Health Services, Mackay; Queensland Injectors Health Network (QUIHN), Brisbane, Gold Coast, and Sunshine Coast; Kobi House, Toowoomba; West Moreton Sexual Health Service, Ipswich; Drug and Alcohol Services South Australia, Adelaide; Hindmarsh Center, Hindmarsh; Nunkuwarrin Yunti Community Health Center, Adelaide; South Australia Voice for Intravenous Education (SAVIVE); AIDS Council South Australia, Norwood; Parks Community Health Service, Adelaide; Port Adelaide Community Health Service, Port Adelaide; Noarlunga Community Health Service, Adelaide; Northern Metropolitan Community Health Service NSP and Shopfront, Salisbury; Devonport Community Health Center, Devonport; Launceston Sexual Health, Launceston; Tasmanian Council on AIDS, Hepatitis and Related Diseases (TasCAHRD), Hobart and Glenorchy; The Link Youth Health Service, Hobart; Health Works, Footscray; Melbourne Inner Needle Exchange, Collingwood; South East Alcohol and Drug Service, Dandenong; St. Kilda NSP; Southern Hepatitis/HIV/AIDS Resource and Prevention Service (SHARPS), Melbourne; WA AIDS Council Mobile Exchange, Perth; Western Australia Substance Users Association (WASUA), Northbridge and Bunbury.

Keywords:

behavioral risk factor surveillance system; intravenous substance abuse; needle exchange programs; sentinel surveillance; viral infection

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