2 The Epidemiology of Injection Drug Use
Estimating the numbers of injection drug users in the United States is an important part of planning the nation's response to the public health threat posed by the AIDS epidemic. At federal, state, and local levels, policy planners need reasonably accurate estimates regarding the size, characteristics, and geographical distribution of the drug-injecting population within their jurisdictions in order to establish a sound rationale for allocating limited resources. Although there have been some advances, it remains true that precise estimation of the numbers of injection drug users remains extraordinarily difficult. Given the illegal and covert nature of the behavior, the situation could hardly be otherwise.
A critical issue in counting injection drug users is definitional: injection drug users vary greatly in the type and combinations of drugs they use, the frequency of injection, the settings in which injections occur, the amount of sharing of injection equipment that takes place, and so forth. The National Research Council dealt with this issue in a previous report, AIDS, Sexual Behavior, and Intravenous Drug Use (Turner et al., 1989).
One particularly important example concerns the injection of cocaine. Because of the shorter duration of euphoria associated with this drug, users who inject cocaine tend to go on binges or "runs," in which they inject repeatedly in a relatively short time; that is, cocaine is sometimes taken in discrete episodes of high-intensity use (Gawin and Kleber, 1985). A binge episode may be followed by an extended period of no injection at all. This pattern is quite different from the typical pattern for a heroin injector, who
may inject a few times throughout the day to forestall physical withdrawal. The cocaine user on a run may be more likely to share needles, due to the need for repeated administration of the drug within a short time in order to satisfy the intense craving to consume more of the drug. This illustrates the point that merely counting the number of injection drug users, defined in some fashion, may not be sufficient to adequately address the prevention needs of certain subgroups of injection drug users.
The National Research Council report found "the current estimates of the prevalence of IV drug use to be seriously flawed" (p. 233) and recommended that a high priority be given to research on the estimation of the number of injection drug users in the United States (Turner et al., 1989). Since that report, a special issue of the Journal of Drug Issues (Hser and Anglin, 1993) on prevalence estimation techniques for drug-using populations makes note of improvements in the quality and quantity of data available and convincingly documents a growing recognition of the importance of this area of inquiry. Acknowledging the imprecision of existing estimation models, Anglin et al. (1993) are more positive about the state of the field. By looking at prevalence estimation as policy research, they stress the potential of this work to inform ongoing decision making, thereby emphasizing appropriate but cautious application of estimates with less stress on absolute precision. In sum, although the continuing imprecision in estimates of the number of injection drug users remains a concern, this panel recognizes the need to make the best use of available data to inform the decisions of policy makers.
ESTIMATES OF INJECTION DRUG USERS
Despite the imprecision of available estimates in this country, most would agree that the magnitude of the population of drug injectors is a major influence on evolving patterns of HIV transmission. U.S. Public Health Service reports dating back to the 1980s all estimate the national figure for injection drug user to exceed 1 million. The Centers for Disease Control (1987a, 1987b) reported approximately 1.5 million injection drug users for 1986 and a revised figure of 1.1 million for 1987. One year later, the National Institute on Drug Abuse (NIDA) estimated that there were about 1.1 to 1.3 million injection drug users in the United States (Schuster, 1988). In its 1989 report, the National Research Council included a critique of the estimation methods used to generate these figures, suggesting that it was not unreasonable to believe that their margin of error could be as great as 100 percent (Spencer, 1989). The report cautions, "That is to say, the true number of IV drug users could be as few as half a million or as great as 2 million" (Turner et al., 1989:230).
Scott Holmberg (1993, 1994) of the Centers for Disease Control and
Prevention has recently attempted to estimate the number of "current active" injection drug users in the 96 largest metropolitan statistical areas (MSAs) in the United States—those with more than 500,000 inhabitants. Holmberg's estimated number of injection drug users in these 96 MSAs is approximately 1.5 million, which is in line with other estimates. Although the 96 MSAs have only about 160 million persons, or about 62 percent of the entire U.S. population of 260 million, it is likely that they include a much higher percentage of all injection drug users. Holmberg (1993, 1994) notes that these areas include about 85 percent of all reported AIDS cases, and it is likely that they include approximately the same proportion of injection drug users.1 Thus, if at least 85 percent of all injection drug users are located in the 96 MSAs, that suggests a national total of injection drug users on the order of 1.7 million. Holmberg's detailed procedure for estimating the number of injection drug users in these metropolitan statistical areas consisted of compiling available information from diverse sources (i.e., published and unpublished). Specific studies of injection drug users performed by federal agencies, health departments, drug treatment services, academic institutions, and a large government contractor (i.e., Research Triangle Institute) were reviewed to arrive at estimates for individual MSAs. Holmberg's procedure included several criteria to ascertain the reasonableness of his estimates: (1) the estimated number of injection drug users should be consistent with the proportion of known injection drug users in treatment (reflecting NIDA estimates of the percentage of injection drug users in treatment); (2) the number should likewise correspond to the proportion of known injection drug users tested for HIV at confidential counseling and testing sites, where such information was available; (3) the estimated proportions of injection drug users should be consistent with the stratification of high-, medium-, and low-injection-use cities, as defined by the Drug Abuse Warning Network (DAWN) data, treatment providers, and ethnographers.
With respect to regional and geographic variation, the pattern is one of clusters of injection drug users, and the clusters are well dispersed across the country. Half of all injection drug users (in the 96 largest MSAs) are found in the top 16 MSAs. Of the top 11 MSAs, 6 are part of the Northeast corridor: Boston, New York City, Newark, Philadelphia, Baltimore, and Washington, D.C. Nearly a quarter (23.5 percent) of all injection drug users (in the 96 largest MSAs) are located in just these 6 MSAs. The other three-quarters are located in the other 10 MSAs: Los Angeles, Chicago, Houston, Detroit, Miami, Riverside, San Francisco, Atlanta, San Juan, and Fort Worth. Three of these are on the West Coast, two in the upper Mid-west,
two in Texas, two in the South, and the other in San Juan, Puerto Rico. Thus, it is clear that injection drug users are geographically dispersed throughout the nation, notwithstanding the density in the Northeast corridor.
Few national estimates of injection drug users as geographically specific as those provided by Holmberg have been published, making comparisons difficult; however, one estimate is that provided for New York City. In a paper commissioned by this panel, Frank and Williams (1994) review the history of narcotic use and prevalence estimation in New York. A narcotics registry was maintained in New York City from 1965 to 1974 in an attempt to approximate a census of addicts. During this period, almost 900,000 reports were recorded on nearly 300,000 individuals. More than 95 percent of the individuals reported were using heroin, which at the time was considered almost tantamount to being identified as an injector. Utilizing the registry as a foundation for the years 1970 to 1974 and then developing a synthetic estimation model to project for later years, Frank and Williams estimated the numbers of narcotic abusers in New York City to be 165,000 in 1970 and 200,000 in 1980. The authors suggest that the number of heroin injectors in New York City is now probably less than the 1980 figure of 200,000 due to AIDS mortality and the growing popularity of snorting heroin as a route of administration, but no more recent prevalence estimates have been attempted. The authors do not account for the possible shift in type of drug injected (i.e., cocaine), which might be missed by their methodology. Nonetheless, the Frank and Williams estimate is in accordance with Holmberg's New York City figure of 168,300.
National Surveys of Drug Abuse
Another source of information about the size of the injection drug use population is national surveys of substance abuse.
National Household Survey on Drug Abuse
The National Household Survey on Drug Abuse (NHSDA) of the Substance Abuse and Mental Health Services Administration (SAMHSA) provides an important source of data on drug use among the general population, although it cannot estimate the entirety of the injection drug user population in the country. In particular, it has major problems as a precise basis for estimating numbers of hard-core drug users, including injectors (National Institute on Drug Abuse, 1994a; U.S. General Accounting Office, 1993). The representative sample of households excludes injection drug users who are homeless, institutionalized, or transient, and the injection drug users who are approached may be reluctant to participate. Also, the
survey relies on self-reports, and the stigma associated with drug injection may contribute to greater denial on the part of those who do participate.
Given the fact that the NHSDA is widely acknowledged to underrepresent injection drug users, it is surprising to note the magnitude of the subpopulation it has uncovered.2 The 1991 survey revealed an estimated 3,768,000 people who have used a needle to inject a drug at some time in their lives and 1,083,000 who have done so in the past year. The 1992 estimated figures were much lower: 2,984,000 for lifetime prevalence and 659,000 for the past year. With regard to age, in 1991, 29 percent of those who reported having injected drugs in the past year were between the ages of 12 and 25 (Table 2.1). In 1992, that age group represented 44 percent of those who had reported injecting drugs in the past year.
Because the populations sampled in these 2 years were highly similar, the observed drop in lifetime prevalence and observed fluctuations by other demographic characteristics cannot be credibly attributed to the relatively minor changes in the sampling frame from one year to the next. As a consequence, this anomaly helps to underscore the imprecision with which the NHSDA is able to estimate numbers of injection drug users.
The large fluctuations in the NHSDA estimates are due in part to the nature of the targeted population. That is, the survey is designed to capture drug use among households nationwide, not specific population subgroups such as injection drug users. As a result, there are too few members of certain specific subgroups (e.g., injection drug users) to allow for the computation of reliable estimates. This is reflected in the large standard errors associated with the estimated number of injection drug users reported above.
Washington DC Metropolitan Area Drug Study
The Washington DC Metropolitan Area Drug Study (DC*MADS) was funded by the National Institute on Drug Abuse (1993, 1994b, 1994c) in an attempt to examine the nature and extent of drug abuse among all types of people residing in a single metropolitan area, with a special focus on populations that were underrepresented or unrepresented in the NHSDA (National Institute on Drug Abuse, 1994b). These special populations, including homeless people, transients, and institutionalized individuals, represent people who tend to be at risk for drug abuse.
Of particular relevance to the topic of this report, DC*MADS estimated that adding homeless and institutionalized populations to the 1991 NHSDA population of past-year injection drug use in the D.C. MSA would have increased the estimate of currently active injection drug users from 0.20 to 0.30 percent, an increase of one-third. The number of injection drug users among the population of households was estimated at 5,987; including the homeless, transient, and institutionalized populations, the number was 8,740.
TABLE 2.1 Needle Use: Ever Used and Past Year Use by Age, Sex, and Race, 1992 and 1991 NHSDA Dataa
(Needle use is defined as the injection of cocaine, opiates, or psychotherapeutics for nonmedical reasons at least once in the previous 12 months.) This difference is too small to change prevalence estimates noticeably, but it would result in a one-third increase in the population estimates often used by providers for estimating the number of people in need of treatment and, for that matter, HIV prevention services.
One noteworthy point is that most of the injection drug users were found in the household population (about two-thirds). It is of course the case that some unknown portion of injection drug users either denied such
use or refused to participate in either the NHSDA or the DC*MADS studies. Thus, these estimates of one-third and two-thirds have some unknown degree of associated error.3
Surveys of Youth
NIDA's Monitoring the Future survey provides the most accurate data available on drug use among secondary school students, but it has the same limitations as the NHSDA in its ability to estimate the hard-core subpopulation of drug users that includes injection drug users. However, like the NHSDA, the Monitoring the Future survey does pick up a small but substantial number of its sampled population who acknowledge having injected drugs. In 1992, 1.7 percent of high school seniors reported having injected a drug during their lives, and 0.8 percent did so in the past year. In 1993, 1.4 percent reported having injected during their lives, and 0.7 percent did so in the past year (O'Malley, unpublished data).
The Centers for Disease Control and Prevention conducted several nationally representative surveys of students in grades 9-12 asking about various risk behaviors, including injection of drugs (Centers for Disease Control and Prevention, 1992). The 1990 Youth Risk Behavior Survey showed 1.5 percent lifetime prevalence for grade 10, and 1.3 percent for grade 12, which are fairly close to the corresponding figures from the Monitoring the Future surveys (1.4 percent and 1.7 percent, respectively). Another noteworthy finding is that studies in both San Francisco and New York report that sharing appears to be more frequent among younger injectors (Guydish et al., 1990; Kleinman et al., 1990), who are typically not very well represented in current needle exchange programs in this country (see the section on the demographics of program participants in Chapter 3).
In sum, across a number of surveys of the type that would be expected to underestimate injection drug use, there is a fair amount of consistency, suggesting a prevalence rate that is clearly not zero and may be as high as 1 to 2 percent among young Americans. None of the surveys show any dramatic shifts in recent years.
Another perspective that is important to consider in light of broader population-based surveys of injection drug users is the limited number of injection drug users that are in treatment. It is estimated that between 10 and 20 percent of injection drug users are in drug treatment at any give time (Centers for Disease Control and Prevention, 1990; Office of Technology Assessment, 1990; Wiley and Samuel, 1989; Schuster, 1988).
CHARACTERISTICS OF INJECTION DRUG USERS
Given the substantial uncertainty in our ability to specify numbers of injection drug users in the United States, it is likewise problematic to estimate accurately the characteristics of this elusive population. Yet existing surveys, drug abuse indicator databases, and other research offer considerable insight in beginning to identify the composition of injection drug users within our borders.
A review of available data reveals at least two predominant themes. First, the overall population of injection drug users in the United States is quite heterogeneous in composition. It is emphatically not the case that all injection drug users are male minorities located in large urban areas, as stereotypes would imply. Second, the composition and characteristics of this population are continually evolving and have changed markedly over time. These facts suggest that the useful improvements that can be made in prevalence estimation techniques are not limited merely to reducing the margins of error for a national approximation of numbers of injectors. To offer meaningful insight, future estimation models will also need to reflect important regional differences and be generated at regular intervals so as to account for variation over time.
A National Profile
NIDA's National AIDS Demonstration Research (NADR) programs provide the most comprehensive profile of active drug injectors not in drug abuse treatment in this country (Brown and Beschner, 1993). However, their description of the characteristics of injection drug users is based on a large sample of out-of-treatment injectors, so the information may be limited by a volunteer/selection bias. As noted earlier in this chapter, it is extremely difficult, if not impossible, to get a truly representative sample of injection drug users. The most feasible approach to generating sound inferences is to obtain information from multiple sampling schemes, which includes the efficiency of sampling entrants to drug treatment programs and entrants into the criminal justice system, as well as community-based recruitment schemes, as was done for NADR. Each scheme has limitations, but in combination they allow for sound inferences (Vlahov and Polk, 1988; Alcabes et al., 1991, 1992; Watters and Biernacki, 1989). In particular, surveys of entrants into treatment for drug abuse at multiple sites (Hahn et al., 1989; Prevots et al., 1995; Battjes et al., 1994) and prison-based surveys at multiple sites (Vlahov et al., 1991a) show similar patterns of geographic diversity of risk behaviors and HIV seroprevalence, which helps to validate estimates and inferences derived from the NADR database.
In the NADR study, a review of the sociodemographic characteristics
of 13,475 active injection drug users from 28 sites across the country, the diversity of this population begins to emerge. Just over half (51 percent) of the injection drug users were African American, but a cross-site comparison indicated a range from 9 to 95 percent African American. One-quarter were Hispanic (ranging from 0 to 81 percent by site), and 22 percent were white (ranging from 3 to 65 percent by site). The percentage of females was 26 percent and ranged from 12 to 37 percent by site. The percentage of high school graduates was 45 percent and ranged from 31 to 67 percent by site. The percentage unemployed was 55 percent and ranged from 29 to 75 percent. The percentage that had previously been in jail was 81 percent and ranged from 64 to 94 percent. The percentage that had previously been in substance abuse treatment was 59 percent and ranged from 29 to 75 percent). The primary drugs injected were heroin (28 percent), cocaine (21 percent), and the combination (speedball) of heroin with cocaine (35 percent); these proportions varied substantially among sites, with 6 to 57 percent injecting heroin alone, the same range injecting cocaine alone, and 6 to 75 percent injecting the combination of heroin with cocaine (all data from Brown and Beschner, 1993:529).
Further analysis of the NADR database offers additional insight into patterns of drug use and high-risk behaviors for the transmission of HIV among injection drug users. For 25,603 members of the sample, half reported injecting daily (p. 118). Daily injection among African Americans and Hispanics was similar at 45 and 46 percent, respectively, and highest among whites at 63 percent. The difference in frequency of daily injection by gender was negligible.
An examination of types of sharing behaviors that place injectors at risk of HIV infection was conducted on 17,891 injection drug users in the NADR database who had reported a history of needle sharing (pp. 124-125). The most frequently reported risk behaviors included sharing cookers for the preparation of injectable solutions and sharing the rinse water used to flush syringes following injection, 90 and 78 percent, respectively. Although both these practices are considered to be of lesser risk than the sharing of syringes, it is notable that neither practice would be directly addressed by syringe exchange, and bleach distribution would be of value only as a potential means to decontaminate shared cookers. The greater risk behavior of reusing needles was reported by 68 percent of the sample, and 41 percent had rented needles in the past.
With regard to types of needle-sharing partners among 18,918 members of the NADR sample (p. 122), 68 percent reported having shared with friends, 67 percent with ''running partners," 52 percent with a spouse or partner, and 25 percent with strangers.
Although none of the NADR sites purport to have recruited a representative sample of injection drug users from the regions they covered, it is
interesting to note that, by focusing on active injectors not in treatment, this program exclusively enrolled members of the hard-core drug-using population, which the major household and student surveys underrepresent. Nevertheless, the fact that the mean age of subjects in the 28-site NADR analysis ranged from ages 31 to 40 and the mean years of injection drug use ranged from 10 to 19 suggests that this database overrepresents the most long-standing and active subpopulation of hard-core injection drug users. As a consequence, it appears that NADR was more successful in recruiting well-established social networks of injection drug users than recent initiates or occasional users. Despite these limitations, the NADR study is important because it shows that a large proportion of injection drug users had no history of drug abuse treatment, despite mean duration of injection of 10 to 19 years. This suggests that there is a subgroup of injection drug users not being accessed into treatment. However, a substantial proportion of these high-risk injection drug users do have a repeated history of treatment, suggesting that treatment as an HIV prevention activity may need to be supplemented (Siegal, 1995).
Additional studies suggest that there may be a secular trend toward reductions in high-risk injection practices. Several studies have reported decreases in risk over time (Battjes et al., 1992; Selwyn et al., 1987; Celentano et al., 1991). For example, Vlahov et al. (1991c) studied an out-of-treatment sample of injection drug users in Baltimore. They focused on behaviors during the 3 months following injection incidence, examining time trends from 1982 through 1987 among successive cohorts of injection drug users classified according to the year in which they first injected. They found significant increases over time in the proportion who sometimes used sterile needles, a decrease in the proportion always using equipment that had previously been used by others, and a decrease in the number of needle-sharing partners. Another relevant finding was a dramatic shift in the first drug injected from heroin to cocaine between 1982 and 1987.
Local Drug Use Trends and Patterns
As succeeding chapters of this report will make clear, a critical aspect of drug use is that it is specific to regions and communities. Although its reports do not focus upon injection drug use alone, the proceedings of NIDA's Community Epidemiology Work Group (CEWG) further illustrate the diversity of injection drug-using patterns across the country and the extent to which these patterns change over time. The CEWG is a drug abuse surveillance network composed of researchers from 20 major U.S. metropolitan areas: Atlanta, Boston, Chicago, Dallas, Denver, Detroit, Honolulu, Los Angeles, Miami, Minneapolis, Newark, New Orleans, New York, Philadelphia, Phoenix, St. Louis, San Diego, San Francisco, Seattle, and
Washington, D.C. Its primary mission is to provide a semiannual community-level assessment of drug abuse, principally through the collection and analysis of epidemiologic and ethnographic research data. It provides current descriptive and analytic information on the nature and patterns of drug abuse, emerging trends, consequences of drug abuse, and characteristics of vulnerable populations.
Every six months, the work group meets to share information on recent trends. Sources of data include:
reports from researchers, often ethnographic, in major metropolitan areas about local situations,
DAWN data on drug-related deaths, as reported by medical examiners,
DAWN emergency room reports of drug-related medical emergencies, treatment admissions data where available,
data from the Drug Enforcement Administration (DEA) Domestic Monitor Program drug intelligence reports on seizure, price, purity, prescription, distribution, and arrests,
results of urinalysis data from the Drug Use Forecast program sponsored by the National Institute of Justice, and
data on HIV and AIDS from the Centers for Disease Control and Prevention.
The various sources are examined by locality and semiannual reports discussing the trends are produced.
The CEWG's greatest strength is its ability to monitor drug-use trends and document regionally specific patterns of drug use. Helping to improve our understanding of injection drug use, its reports include the history of the shift from heroin alone as a primary drug of injection in the late 1970s to the integration of cocaine during the 1980s, both as an independent drug of injection and as used in combination with heroin. Also reported was the emergence and eventual decline some 10 years ago of a midwestern epidemic of pentazocine and tripelennamine (Ts and blues) injection. Whereas the injection of stimulant drugs other than cocaine has remained a relatively isolated phenomenon across much of the country, a series of CEWG publications reveals that amphetamine injection has been a major problem in San Diego, and the injection of stimulants in Chicago is much less common and typified by injection of the pharmaceutical drugs phenmetrazine among whites on the north side of the city and methylphenidate among African Americans on the city's south side. Thus, although not directly involved in the calculation of drug-use prevalence estimates, it can be seen that the CEWG is an invaluable resource in tracking the nature and scope of substance abuse across major metropolitan areas.
Given our lack of ability to definitively describe the extant population of injection drug users, it should be clear that any attempts to project into the future are highly speculative. Yet drawing some sense of factors that may influence the size of the injection drug user population in the coming years seems warranted. In referring to CEWG, trend analysis would seem to hold some promise as a basis of forecasting; however, many of the secondary indicator databases relating to substance abuse do not currently report on route of drug administration. As a consequence, CEWG does not currently monitor trends in injection drug use as an independent focus of analysis. Recent proceedings nevertheless include discussion of at least two trends, previously noted by Frank and Williams (1994), that may significantly influence future injection drug user prevalence. The first is AIDS-related mortality, which has already begun to deplete the numbers in this population. The second, and potentially countervailing trend, is a resurgence in the prevalence of heroin use, which has been associated with an increasing number of initiates who are using the drug intranasally.
The proceedings of the CEWG meeting in December 1993 show the trend of increasing intranasal heroin use to be most pronounced in Newark, Chicago, and New York City (National Institute on Drug Abuse, 1994a). Newark reported 66 percent of heroin admissions to be snorters (French and Mammo, 1994). In Chicago, intranasal consumption accounted for 60 percent of treatment admissions for heroin dependence (Wiebel et al., 1994), and in New York City, 51 percent of heroin admissions reported a primary intranasal route of drug administration (Frank and Galea, 1994). DEA's Domestic Monitor Program (Drug Enforcement Administration, 1994), which analyzes the price and purity of heroin in major metropolitan areas, has documented substantial increases in the purity of heroin sold on the streets over the past half decade. Some have noted a direct association between the availability of higher-quality heroin and its intranasal consumption (Ouellet et al., 1993; Des Jarlais et al., 1994; Friedman et al., 1994; Frank and Williams, 1994); however, it is not yet clear whether this regionally emergent epidemic will spread across the entire country. It is also uncertain as to the proportion of current intranasal heroin users who are or will become physically dependent, or the numbers likely to ultimately progress to injecting the drug. Ouellet et al. (1993) in Chicago noted that current younger snorters of heroin do not see themselves as being at any risk for progressing to injection, unlike older injectors, who consider intranasal heroin use as merely a transitional phase leading up to injection. As cautioned by Friedman et al. (1994) in a commissioned paper for this panel on the etiology of drug injection, "Heroin snorters represent a real but unknown risk for progressing to injection drug use."
As some have suggested (Anglin et al., 1993), accurate estimates of injection drug use may be less important to policy makers than a constellation of data that helps to assess need and allocate limited resources. If this is the case, then more systematic efforts need to be directed toward monitoring patterns and trends in injection drug use, including improvements in recording the route of administration in secondary drug abuse indicator reporting systems. NIDA's Community Epidemiology Work Group provides one mechanism that may prove useful, particularly if expanded to incorporate more specific data on route of administration in its monitoring purview. This would provide policy planners with periodic updates as to whether the pool of injector drug users is likely to increase or decrease and whether there are any substantial shifts in the composition of the pool, including the emergence of injection as a route of drug administration among any previously unafflicted subpopulations.
The panel recommends that:
The Assistant Secretary for Health should charge appropriate agencies (i.e., the National Institutes of Health and the Centers for Disease Control and Prevention), in consultation with academic departments of epidemiology, to develop more effective surveillance of drug use, particularly for local areas. The data collected should move beyond gross prevalence estimation of drug use and toward detailed information about users. This should include data on behavioral dynamics (e.g., pattern of drug use, sharing of drugs and drug paraphernalia, social context of drug use) by drugs of choice, routes of administration for each, and the flow of injection drug users into and out of drug treatment programs.
Note that the DC*MADS survey did not cover the same geographical area as covered by Holmberg's review, which precludes a direct comparison between the two estimated numbers of injection drug users.
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