This article was downloaded by: [Florida International University]On: 11 May 2014, At: 05:42Publisher: RoutledgeInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

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Using Knowledge of Social Networks toPrevent Human Immunodeficiency VirusInfections: The Colorado Springs StudyWilliam W. Darrow Ph.D. a , John J. Potterat b , Richard B.Rothenberg c , Donald E. Woodhouse d , Stephen Q. Muth b & AldenS. Klovdahl ea Florida International University , USAb El Paso County Department of Health and Environment , USAc Emory University School of Medicine , USAd Lock Haven University , USAe The Australian National UniversityPublished online: 19 Nov 2012.

To cite this article: William W. Darrow Ph.D. , John J. Potterat , Richard B. Rothenberg , DonaldE. Woodhouse , Stephen Q. Muth & Alden S. Klovdahl (1999) Using Knowledge of Social Networks toPrevent Human Immunodeficiency Virus Infections: The Colorado Springs Study, Sociological Focus,32:2, 143-158, DOI: 10.1080/00380237.1999.10571132

To link to this article: http://dx.doi.org/10.1080/00380237.1999.10571132

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USING KNOWLEDGE OF SOCIAL NETWORKS TO PREVENT

HUMAN IMMUNODEFICIENCY VIRUS INFECTIONS:

THE COLORADO SPRINGS STUDY

WILLIAM W. DARROW SOCIOLOGICAL FOCUS Florida International University Vol. 32 No. 2

JOHN J. POTTERAT May 1999 El Paso County Department of Health and Environment

RICHARD B. ROTHENBERG Emory University School of Medicine

DONALD E. WOODHOUSE Lock Haven University

STEPHEN Q. MUTH El Paso County Department of Health and Environment

ALDEN S. KLOVDAHL The Australian National University

The development of an applied social science research, project with implications for the management of infectious diseases is described. The project evolved nut of a multidisciplinary effort to understand how sexually transmissible agents enter into social systems, are transmitted and can be identified, prevented and controlL'd. Collaborators began their investigations in the 1970s by looking at the social and behavioral patterns of individuals infected with gonorrhea, then applied similar methods in the late 1980s and early 1990s to examine the ways human immunodeficiency virus (HIV), the pathogen that causes AIDS, might spread among heterosexuals in a mid-sized American community. Concepts of personal and social networks of individuals linked by social, sexual and drug-sharing exposures guided the systematic collection of information from 595 participants in a large, prospective study. Results suggest that HIV did not spread among heterosexuals who engaged in risky sexual and needle-sharing behaviors in Colorado Springs because those infected with HIV (and capable of infecting others) were located in small, relatively isolated network components or in peripheral network regions.

An earlier version of this report was presented by William W. Darrow, Department of Public Health in a Thematic Session on Networks, Norms, Community and AIDS at the Annual Meeting of the American Sociological Association in Washington, DC, on August 20, 1995. Support for our research has been provided by the Centers for Disease Control (U64/CCU802975) and the National Institute on Drug Abuse (ROI DA09928-01A1). We gratefully acknowledge the contributions of many colleagues and friends in Colorado Springs, especially Tammy S. Maldonado, John B. Muth, M.D., Lynanne Plummer, R.N., Judith U. Reynolds, M.D., Helen L. Zimmerman-Rogers, Helen P. Zimmerman, R.N. and the 595 participants in our study who welcomed us into their lives. Please address correspondence to: William W. Darrow, Ph.D., Professor, Department of Public Health, Florida International University, 3000 151st Street, AC1-394F, North Miami, Florida 33181-3600 Tel. 305 919-5252; Fax: 305 919-5507; E-mail: darrowwefiu.edu.

We also thank Vladimir Batagelj and Andrej Mrvar for use of their freeware package "Pajek" in creating Figure 1 (http://vlado.faf.uni-h'.si/pub/networke/pajek/).

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144 SOCIOLOGICAL FOCUS

INTRODUCTION

J? undamental questions in sociology include: (1) How do groups form? (2) How are groups maintained? (3) How do groups dissipate and vanish? The smallest group to study is the dyad, or couple. Attraction may bring two people together for a number of reasons, including a moment of sexual bliss. That moment could seal a bond that will last for a lifetime. That moment may also lead to the transmission of a life-threatening sexually transmitted disease (STD). As a consequence of sexual encoun­ters, infectious organisms can be passed from an infected host to a susceptible partner. The purpose of our report is to describe the origin and development of an applied social research project firmly grounded in a longstanding curiosity in these funda­mental social processes: (1) How do sexual partnerships form? (2) How long do they last? (3) What are the consequences of sexual partnerships for social life in contem­porary American communities?

HISTORICAL BACKGROUND

The Venereal Disease (VD) Branch of the Communicable Disease Center (CDC) began to look at new opportunities to prevent and control sexually transmitted infec­tions (STIs) other than syphilis in the early 1970s. First of all, technological advances, such as the development and distribution of Thayer-Martin culture medium (Darrow and Bradford 1977), made it possible to detect Neisseria gonorrhoeae more easily than in the past. Secondly, the availability and adoption of improved diagnostic tests for viral (e.g., hepatitis B) as well as bacterial (e.g., gonorrhea) pathogens enabled epidemiologists to define and characterize the extent and seriousness of a variety of STIs. Thirdly, the retirement of the Chief of the VD Branch at CDC in Atlanta, Georgia, in 1971, combined with disclosures about the infamous "Tuskegee Study of Untreated Syphilis in the Negro Male" the following year, created opportunities for young and talented Public Health Service (PHS) officers to work on venereal problems other than syphilis, the major focus of PHS concerns from the establishment of a VD Division in the Department of the Treasury in 1919 until 1972.

To prepare for innovative STD prevention initiatives, a venereal disease control strategic planning system (VDCSPS) was established in the newly re-structured VD Control Division at CDC (Rothenberg, Judson and Maltz 1979). Two field stations were created to gather baseline data and to prepare for field trials of time-honored and innovative interventions. Dr. James W. Curran, who later chaired the CDC's Task Force on Kaposi's Sarcoma and Opportunistic Infections in the early 1980s, was assigned to the State Department of Health in Columbus, Ohio, to organize STD research efforts there. Dr. Richard B. Rothenberg was recruited to set up similar activities in Colorado.

Although the two field sites pursued similar interests, the site in Ohio empha­sized clinic-based interventions and decided to focus on the serious problem of pelvic inflammatory disease in young women. The site in Colorado was interested in exam­ining traditional VD control concepts and strategies, such as "contact tracing* (US DHEW 1962) and, therefore, began to look more closely at how to stop the spread of STIs in the community. As the sole social scientist in the VD Control Division, William W. Darrow was asked to support VDCSPS field operations. In 1974, Rich

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USING KNOWLEDGE OF SOCIAL NETWORKS TO PREVENT HIV INFECTIONS 145

Rothenberg invited Bill Darrow to meet with him and John Potterat, Director of VD Control for the El Paso County (Colorado) Department of Health and Environment, to discuss the role of prostitutes in the transmission of gonorrhea and other STIs. That's how our relationship was first established and our studies of STIs — that have now continued for over 20 years — began.

STREET PROSTITUTES AND OTHERS INFECTED WITH GONORRHEA

As we began to study the characteristics of people infected with gonorrhea, it became apparent that a relatively small number of individuals was involved in chains of heterosexual disease transmission. In Colorado Springs, three categories of people seemed to account for a majority of gonococcal infections: servicemen stationed at a nearby military installation, other men out for a good time (e.g., "cowboys") and their female sex partners. Women involved in gonorrhea transmission tended to be street prostitutes or nonprostitutes who had multiple sex partners over a relatively short period (e.g., 30-90 days).

To develop appropriate social or behavioral interventions for women likely to transmit gonorrhea and other STIs, we decided to do in-depth interviews with 15 street prostitutes and 15 non-prostitutes matched for race/ethnicity and age group (Potterat et al. 1985a). Both groups consisted of women who had visited the County STD clinic within the past year and had been found to be infected with gonorrhea. In addition, comparison women invited to participate in this study had named two or more male partners who were found to be infected with gonorrhea.

We found very few differences in the characteristics and experiences of the prostitutes and 15 comparison women. Both groups frequently reported childhood abuse, sexual victimization and difficulties in school, with family members and with other social institutions. Although most of our respondents seemed susceptible to entering prostitution (based on their reports of lifetime experiences), those who had engaged in prostitution more often reported exposures to others (usually — but not always — men) who encouraged them to do so. Our findings suggested that public health efforts to prevent the spread of gonorrhea in Colorado Springs should focus on both groups of women because both appeared to serve as bridges linking a population of highly mobile servicemen with the slightly more stable sexually active civilian population.

THE CONCEPT OF A CORE GROUP At the same time CDC was supporting field studies in VDCSPS sites, Yorke,

Hethcote and Nold (1978) were supported by CDC in their efforts to develop a mathematical model for the spread of gonorrhea in a community. Their deterministic model suggested that a relatively small number of very sexually active people was responsible for generating the majority of gonococcal infections. These individuals were infected frequently and transmitted their infections to others who, in turn, spread it to others. Since gonorrhea was endemic' and concentrated in a small population at the center of epidemic outbreaks, Yorke (who later became famous for his work on chaos theory), Hethcote and Nold labeled this postulated population "the core group." Subsequent studies in Colorado Springs (Potterat et al. 1985b) and

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146 SOCIOLOGICAL FOCUS

elsewhere (Rothenberg 1983; Garnett and Anderson 1993) appeared to provide empirical evidence for the presence of a core group in community outbreaks of gonorrhea and other STIs (Gunn et al. 1995).

HETEROSEXUAL TRANSMISSION OF HUMAN IMMUNODEFICIENCY VIRUS

In spring 1981, CDC began to receive reports of a highly unusual and apparently acquired immune deficiency syndrome (AIDS) in men who had sex with men (Darrow 1991). A case-control study (CDC AIDS Project 1) suggested that AIDS was caused by an infectious agent that was transmitted from one man to another through unpro­tected sexual activities. Additional support for the hypothesis that AIDS was an STI came from a follow-up study of men with AIDS (CDC AIDS Project 6) who could be linked as sexual partners (Auerbach et al. 1984). Shortly thereafter, Human Immuno­deficiency Virus (HIV) was identified as the cause of AIDS and an HIV-antibody test was licensed for use in the United States in 1985 to test people for evidence of HIV infection.

If men could infect men with HIV through unprotected sexual activities, it seemed likely that men could also infect women (Friedland and Klein 1987). To examine this possibility, CDC supported a series of epidemiologic studies in the mid-1980s, including a cross-sectional study of the prevalence of— and risk factors for — HIV-antibody in women who had engaged in prostitution (CDC AIDS Project 72). Eight sites were selected to participate in the multicenter study of female prostitutes: northern New Jersey, southern New Jersey, southern Nevada, Atlanta, Miami, Los Angeles, San Francisco and Colorado Springs (Darrow et al. 1990).

The prevalence of HIV-antibody in female prostitutes varied widely in 1986-1987. Street prostitutes in northern New Jersey were far more likely to be infected than brothel employees in southern Nevada (Centers for Disease Control 1987). Major risk factors for HIV infection were a history of injection drug use, a large number of nonpaying partners and having a husband or boyfriend who injected drugs. Prosti­tutes were more likely to engage in unprotected vaginal or anal intercourse with their husbands, boyfriends, or other nonpaying partners than with clients.

THE COLORADO SPRINGS STUDY OF SOCIAL NETWORKS

The cross-sectional, multicenter study of prevalence and risk factors for HIV infection in female prostitutes was followed by a series of prospective studies designed to understand and interrupt heterosexual transmission of HIV (CDC AIDS Project 90). Seroincidence studies with independent protocols were conducted in Atlanta, San Francisco and Colorado Springs. Principal investigators and key staff members met with CDC representatives and consultants (Professors John H. Gagnon, State Univer­sity of New York at Stonybrook and Alden S. Klovdahl, The Australian National University, Canberra) to develop plans for follow-up research projects of prostitutes, their clients and others involved in prostitution.

A social network study of female prostitutes, injection drug users and their named sexual and drug-using partners was designed for implementation in Colorado Springs (Klovdahl et al. 1994; Scott 1991). Anyone who had engaged in prostitution or

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USING KNOWLEDGE OF SOCIAL NETWORKS TO PREVENT HIV INFECTIONS 147

injected drugs within 12 months preceding enrollment was eligible for participation. Recruitment began in 1988 with those most accessible to health department staff: STD, HIV-testing and methadone-maintenance clinic patients.

After confidentiality was assured and informed consent obtained, standardized interviews were conducted to collect sociodemographic, attitudinal, behavioral and personal network information (Woodhouse et al. 1995). Names were sought for the sexual contacts, drug-sharing partners and close personal friends of participants with whom they had shared meals or lodging. If a person was named as a sex or drug-sharing partner by two or more participants (a "cross-link"), we attempted to locate and enroll that person as well. As a result of these procedures, 595 residents of Colorado Springs (1990 population = 397,041) were enrolled over a five-year period of data collection and interviewed at least once.

RESULTS

The 595 participants enrolled between January 1988 and August 1991 identified 10,033 contacts and associates ("nodes" in a network) with whom they had established social, sexual or drug-using connections during the six months preceding interview (Woodhouse et al. 1994). A total of 5,894 (58.7 percent) of these 10,033 contacts and associates were named during initial interviews, an average of 9.9 contacts and associates per person interviewed. These 5,894 contacts and associates, along with the 595 respondents who named them, represented 5,162 different people.

Although there was considerable overlap by eligibility criteria, 197 (33 percent) of the 595 participants were enrolled because they had recent histories of injection drug use, 119 (20 percent) had recent histories of prostitution, 186 (31 percent) had recent social, sexual or needle-sharing exposures with a prostitute or an injection drug user (1DU) and 93 (16 percent) were "cross-links." The 162 male IDUs included 42 men (25.9 percent) who reported sexual contact with women who worked as prosti­tutes; 28 were clients and 14 were "non-paying" partners. Almost half (63, or 47.4 percent) of the 133 women who had engaged in prostitution said they had injected drugs since 1987; an additional 14 women (10.5 percent) admitted to injection drug use before 1987, but not afterwards. Similarly, the 5,162 people named by study participants as their contacts and associates often engaged in sex and drug-using behaviors that are known to transmit HIV.

In spite of high frequencies of injection drug use and unprotected vaginal and anal intercourse, only 17 participants (2.9 percent) tested positive for antibody to HIV (Rothenberg et al. 1995a). Sixteen of the 17 participants infected with the AIDS virus had histories of injection drug use and sharing needles, syringes or other parapher­nalia. Relational data were then used to reconstruct social networks in which partici­pants and their contacts and associates were involved (Klovdahl et al. 1994). Eight of the 17 people infected with HIV were located in a large connected component; a region of a social network consisting of 341 participants linked together by relationships of various kinds, indirectly if not directly. However, seven of these eight HIV-positive participants were found to be peripherally located in this connected region, as measured by their centrality scores (Rothenberg et al. 1995b; Wasserman and Faust 1994). Nine of the 17 were isolated in smaller components (Figure 1). Thus, the lack of heterosexual transmission of HIV observed in Colorado Springs appeared to be

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14H SOCIOLOGICAL FOCUS

FIGURE 1

SOCIAL NETWORK VISUALIZATION SHOWING THE MARGINALIZATION OF HIV-POSITIVE PERSONS WITHIN THE LARGER AND SMALLER

COMPONENTS IN COLORADO SPRINGS, COLORADO, 1988-1992

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^ ί^; ? M ί^: ^ ; ί^ί £-« ί°-°ί fc-i £-? W »_, » _ , » . » ».* ».» ».» ».» * ·» * .»

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USING KNOWLEDGE OF SOCIAL NETWORKS TO PREVENT HIV INFECTIONS 149

associated with the location of potential transmitters outside or at the periphery of a large social network of persons who engaged in risky behaviors (Potterat et al. 1993). The social organization of sexual and drug-using behaviors in this medium-sized American community seemed to keep those who could have transmitted HIV to others at a relatively safe distance.

CENTRALITY AND HIV TRANSMISSION

To explore further the role of social structures in inhibiting (or facilitating) the spread of HIV and other STIs, we compared the characteristics of the 17 HIV-positive participants in Colorado Springs with 46 HIV-negative participants who scored in the top decile for "betweenness" (Freeman 1979) or "information" (Stephenson and Zelen 1989) centrality. For this analysis, only sexual and needle-sharing connections that could have transmitted HIV were considered (for example, see Figure 2). Unlike earlier analyses (described above), information from all face-to-face interviews (595 enrollment interviews, 278 Year-2 reinterviews, 100 Year-3 reinterviews, and 20 Year-4 or Year-5 reinterviews) was included to assess centrality in the largest network component.

The sex ratio for centrally located HIV-negative participants (31 women:15 men) was virtually the inverse of the sex ratio for HIV-positive participants (6 women: 11 men). Ages for the two groups were similar. The mean age of centrally located HIV-negative participants was 29.4 +/- 8.1 years; the mean for HIV-positive participants was 30.6 +/- 5.3 years. Measures of dispersion for age at enrollment were greater for HIV negatives (range, 16-64 years old) than HIV positives (range, 23—41 years old). HIV-positive women tended to be older (28.5+/-4.2) than centrally located HIV-negative women (26.0+/-4.6), but HIV-positive men (31.8+/-5.7) tended to be younger than centrally located HIV-negative men (36.3+/-9.4).

The majority of HIV-positive participants (59 percent) and centrally located HIV-negative participants (59 percent) were white and not Hispanic in origin, but almost two-thirds (64 percent) of men infected with HIV were white while only one-third (33.3 percent) of centrally located HIV-negative men were white (Table 1). Slightly less than half of the 17 HIV-positive and slightly more than half of the 46 centrally located HIV-negative men and women had completed at least 12 years of education when enrolled. Few in either group reported having a full-time job. Infected men said they were employed for 20 hours or more per week in occupations such as roofer, truck driver, telemarketer; fully employed uninfected men centrally located in a large component of interconnected risk takers were working in similar occupations (cab driver, furniture mover, machinist and "process technician for a high-tech electronics firm"). A few uninfected women reported full-time employment as a hair­stylist, waitress and maid, respectively. As shown in Table 1, 18 (60 percent) of 30 uninfected women centrally located in the large component of people at high risk for exposure to HIV derived most of their income from prostitution.

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150 SOCIOLOGICAL FOCUS

FIGURE 2

NETWORK CONNECTIONS OF IV-DRUG USERS IN COLORADO SPRINGS, COLORADO, 1988-1992

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USING KNOWLEDGE OF SOCIAL NETWORKS TO PREVENT HIV INFECTIONS 151

TABLE 1

DEMOGRAPHIC AND SOCIAL CHARACTERISTICS OF 17 HIV-POSITIVE AND 46 CENTRALLY LOCATED HTV-NEGATIVE PARTICIPANTS, COLORADO SPRINGS

Demographic or social characterist ic

White, Non-Hispanic High School Graduate Employed, Full-Time History of Prostitution > 5 years resident in Colorado Springs < 1 year resident a t current address

A110V HIV+ n - 1 7

59 41 18 — 6*

94

P e r c e n t w i t h character i s t i c

- 6 3 ) HIV-

n - 4 6

59 5 9 " 22* — 42* 71*

Female (n - 37) HTV+ n » 6

50 33

0 83

0 100

HIV-n - 3 1

71 48** 10* 97 43* 80*

M a l e d i HIV+ n - 1 1

64 45 27 — 10* 91

- 2 6 ) HTV-7 » - 1 5

33 80 47 — 40 53

one missing observation two missing observations

Only one of the HIV-positive participants had lived in the Colorado Springs area for more than 5 years when enrolled in our study (Table 1). HIV-negative study participants had lived in Colorado Springs longer, but most (58 percent) had moved into the area within the past 5 years. Residential mobility within El Paso County was high for both groups. On average, HIV-positive participants reported living at their current address for fewer than 6 months (mean = 5.8 +/- 5.6 months). Uninfected HIV-negative participants with high centrality scores reported living at their current addresses for slightly longer periods (mean = 17.0 +/- 37.9 months).

Twelve (70.6 percent) of the 17 HIV-infected participants knew from previous HIV-antibody test results that they were infected with the virus that causes AIDS. Thirteen participants were infected with HIV when first tested in Colorado Springs and had moved into the area from Denver (3), New York (2), Puerto Rico (2), New Orleans, San Francisco, Seattle, Pueblo (Colorado), Minnesota and Texas. Four tested negative in Colorado Springs and subsequently tested positive. The four serocon-verters included two men and one woman who reported injecting drugs and sharing needles during the period when they might have become infected with ΗΓν. The other woman said she had engaged in unprotected anal intercourse with her boyfriend, an injection drug user.

Over two-thirds (69 percent) of the HIV-positive participants and almost half (48 percent) of the centrally located HrV-negative participants reported injecting heroin or other drugs into their veins in the past 5 years (Table 2). A majority (64 percent of HIV-positives and 57 percent of HrV-negatives) of IDU reported sharing needles within the last 6 months before enrollment. Quantitative and qualitative data (not shown) suggested that female injectors were only slightly more successful in reducing their needle-sharing practices than men and knowledge of HIV serostatus seemed to have little impact on drug-injection or sexual risk-taking behaviors.

Twenty-six (87 percent) of 30 centrally located HrV-negative women and four (31 percent) of 13 centrally located HDZ-negative men reported having sexual contact with five or more commercial sex partners in the past 5 years (Table 2). Thirty of the HIV-negative women said they had engaged in prostitution; they estimated they had an average of about 1,250 commercial partners in the past 5 years. In contrast, four of

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152 SOCIOLOGICAL FOCUS

five HIV-positive women who said they had engaged in prostitution reported 2, 50, 100 and 1500 commercial partners in the past 5 years. HIV-negative participants also tended to report more non-paying sex partners in the past five years and within the past six months of enrollment than HIV-positive participants. In the past 5 years, female HIV-negative participants reported a mean of 24 non-paying partners, male HIV-negative participants reported a mean of 30 non-paying partners, female HIV-positive participants reported a mean of 7 non-paying partners and male HIV-positive participants reported a mean of 9 non-paying partners. In the past 6 months, HIV-negative women estimated an average (mean) of over 150 sex partners and named an average (mean) of 8.2; HIV-positive women reported an average of 27 and named 1.8.

TABLE 2

DRUG-USING AND SEXUAL BEHAVIORS OF 17 HIV-POSITIVE AND 46 CENTRALLY LOCATED mV-NEGATIVE PARTICIPANTS, COLORADO SPRINGS

Percent with characteristic

Drug-Using or Sexual Behavior

Injected Drug, Last 5 Years Shared Needle, Last 6 Months > 5 Commercial Partners, Last 5 Years > 5 Non-Paying Partners, Last 5 Years > 5 Commercial Partners, Last 6 Months > 5 Non-Paying Partners, Last 6 Months

one missing observation two missing observations three missing observations

a eleven recent IDU (four females and seven males) used in the denominator D twenty-one recent IDU (eighteen females and three males) used in the denominator

Histories of STIs in the groups were similar. Only one participant, an HIV-infected man, reported a history of primary syphilis. Four reported histories of genital herpes infection; all were women and three were not infected with HIV. Three reported histories of other genital ulcer disease; all were men and two tested negative for antibody to HIV. No one reported a history of chancroid. In spite of these histories and perceptions that their chances of being infected with HIV were moderate (27.3 percent) or low (54.5 percent), 32 (71.1 percent) of 45 HIV-negative participants said they had been tested previously for evidence of HIV infection and had received confirmation that they were not infected with the AIDS virus.

When data collection concluded in 1993, two of 31 centrally located HIV-negative women were incarcerated, three could not be found, one was known to have moved out of the area and one had died. Four of the 15 centrally located HIV-negative men were incarcerated, four more could not be located, one was known to have moved elsewhere and one was dead. Two of the six HIV-positive women were in prison; two of the 11 HIV-positive men could not be located, one moved out of town and another was deceased.

All (TV HIV* n - 1 7

69* 64 a

19* 44*

6* 0*

• 63)

mv-η - 4 β

48** 57 b

70*** 59** 52** 11**

Female (n HIV+ n « β

67 25 a

50 50 17 0

1 - 3 7 ) HIV-n - 3 1

58 50 b

87* 55 71 10

Male (n HIV+ n - 1 1

70* 8 6 a

0* 40* 0* 0*

-26) HD/-n - 1 5

23** 100b

31** 69**

8** 15**

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USING KNOWLEDGE OF SOCIAL NETWORKS TO PREVENT HIV INFECTIONS 153

In summary, differences were found in the two groups with respect to sex ratio, the proportion of white males infected with HIV and length of residency in Colorado Springs. Similarities were found with respect to age distribution, levels of educational attainment, current employment status and other lifestyle characteristics. Although HIV-positive residents more often reported a history of injecting drugs, similar proportions of HIV-infected and centrally located HIV-negative participants reported sharing needles. Surprisingly, centrally located HIV-negative participants were more likely than HIV-positive participants to report sexual practices often associated with HIV infection in epidemiologic studies of heterosexual transmission.

CONCLUSIONS AND RECOMMENDATIONS

HIV and other STIs are transmitted from one person to another during close, intimate sexual activities that bring an uninfected and susceptible partner into direct contact with an infected and infectious partner's blood or other bodily fluids that contain infectious agents (Darrow, Jaffe and Curran 1988). Primary prevention efforts in the United States have been devoted to providing accurate information about the ways HIV and other STIs are contracted, so people can make informed decisions about who they will select as their sex partners and how they will engage in sexual intercourse. Recommendations to avoid infection with HIV and other STIs have focused on sexual and drug-using behaviors (Centers for Disease Control 1983; 1986). PHS has recommended: (1) abstinence from drugs and (2) abstinence from sexual intercourse, or (2a) mutual monogamy with an uninfected partner, or (2b) the proper and consistent use of latex condoms until a permanent and uninfected sex partner can be found.

The behavioral patterns of a large proportion of the U.S. population are con­sistent with PHS recommendations (Catania et al. 1995; Laumann et al. 1994). HIV and other STIs are not equal opportunity diseases. Risks for infection are restricted to those who engage in behaviors that expose them to others who harbor infectious agents. Note that risky behavior by itself does not result in an STI or the spread of disease; it is location in social networks of sexually interacting persons in combination with a risky behavior that determines rates of disease transmission. Thus, it is not just what you do that leads to venereal infection, but with whom you do it (Darrow 1976). The social network paradigm advances our understanding of the transmission of HIV and other infectious agents by forcing us to examine people who engage in behaviors that could transmit disease and how these people are linked together in a social system (Klovdahl 1985; Klovdahl et al. 1994).

IMPLICATIONS

Mathematical models, similar to those devised by Hethcote and Yorke (1984) for gonorrhea, have been developed to describe how ΗΓν might spread through human populations (Anderson and May 1988). These models draw attention to three major variables: (1) the number of susceptibles who are exposed to an infectious agent, (2) the risk of transmission for each exposure and (3) the length of time that an infected person can transmit his or her infection to others (Gupta, Anderson and May 1989). They investigate the interaction of groups of people segregated by certain

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characteristics (such as age, sex, ethnicity and risk behaviors) into compartments. Compartments have definable probabilities for sexual interaction; hence, mathe­matical models for HIV transmission are dependent on the estimates of parameters that describe such probabilities, as well as probabilities for exposure and trans­mission. In a multicompartmental model, large numbers of parameters are required. Unfortunately, empirical data for making estimates are often unavailable or unreliable. Nonetheless, such models have considerable heuristic value when contem­plating the natural history of an epidemic, the demographic consequences and potential influences of interventions (Anderson, Gupta and Ng 1990; Blower, van Griensven and Kaplan 1995; Service and Blower 1995).

The social network approach is more empirically based than multicom­partmental models. It tries to place people in their own social settings (apartments rather than compartments). A risk group is a hypothetical construct; the placement of people into risk groups can be misleading (for example, epidemiologists at CDC considered "Haitians" to be a risk group for AIDS in the early 1980s). In contrast, a social network is not an artificial construct, but a description of observed rela­tionships. The contacts, risky and nonrisky, that take place in that context will be modified by it, particularly by the extent to which such contacts propagate infection.

The importance of social network influences on disease transmission can be illustrated by contrasting our findings in Colorado Springs with findings reported elsewhere in the United States. Unlike others studying networks of HIV transmission among IDUs in Baltimore (Latkin et al. 1996a; 1996b), Brooklyn (Friedman et al. 1997; Neaigus et al. 1994) and the South Bronx (Wallace 1988, 1990), we found very little evidence of HIV dispersion among IDUs and their sex partners in Colorado Springs (Potterat et al. 1993; Woodhouse et al. 1994). Many of the elements required for HIV diffusion were present in Colorado Springs, including groups of potential transmitters who injected drugs, shared needles and syringes and frequently engaged in unprotected sexual activities with large numbers of partners. Our analyses of social network and other survey data suggested two possible reasons for the failure of HIV to penetrate the core group and spread from there throughout the community. HIV-infected people were either (1) isolated in small and contained networks with no apparent bridges to larger social networks that could have facilitated transmission or (2) located at or near the periphery of larger networks where risky contacts were more likely to be with other marginalized people rather than more centrally located individuals who could have accelerated HIV transmissions throughout the community.

Debate continues about heterosexual transmission with some believing that HIV will never spread very far in the United States (National Research Council 1993) and others convinced that extensive spread is now in progress (Wallace, Thompson and Flisher 1996; Wallace and Wallace 1997). Our observations in a mid-American community suggest the truth lies somewhere in between. People infected with HIV, including those who inject drugs, are moving around the country and into places like Colorado Springs, but they may not be quickly integrated into existing social networks of IDUs and their sex partners. It takes time for social, sexual and drug-using relationships to form and for social networks to change.

Ironically, social patterns that appear to account for outbreaks of gonorrhea in Colorado Springs (Potterat 1985b) and other communities (Stigum, Falck and Magnus 1994) may be responsible for keeping those infected with HIV who enter into (or pass

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through) these social systems from establishing needle-sharing and unprotected sexual contacts with susceptibles who are central in local networks. In the short term (1988-1993), we found no evidence of extensive spread of HIV among heterosexuals at risk in our community, but the question of long-term effects of social interaction, population mobility and other key variables on heterosexual HIV-transmission patterns remain unanswered (Morris and Dean 1994). Social, sexual and drug-using networks of interest may be unstable (Blower and McLean 1991; Stigum, Magnus and Bakketeig 1997). The potential for future heterosexual HIV epidemics involving hundreds and, perhaps, thousands of people in places like Colorado Springs cannot be dismissed.

APPLICATIONS

As the incidence of reported cases of AIDS appears to be peaking (Lemp et al. 1997) and number of deaths due to AIDS declines (Centers for Disease Control 1997), CDC is still advocating both general and targeted interventions for HIV prevention (Sumartojo et al. 1997) and no one is providing adequate resources for either. Public health infrastructures are collapsing as human populations are being threatened by a variety of infectious agents and other deadly conditions with potentially serious consequences (Garrett 1994). Although some may feel relieved that the threat of a wide-scale HIV epidemic has passed and AIDS can be managed through combination therapies, HIV and other STIs are still being transmitted in American communities and no one in the United States should feel complacent (Institute of Medicine 1997).

Responsible officials should provide renewed support for innovative and traditional public health case-finding methods, such as "contact tracing" and "cluster testing," to obtain essential information about disease transmission patterns for local decision-making (Rothenberg, Potterat and Woodhouse 1996). Furthermore, adequate support should be provided for promising social and other interventions to identify and treat STIs and interrupt disease transmissions in communities (for example, see Broadhead and Heckathorn 1994). Public health workers must be well trained and sensitive to the implications of their data-collection procedures and disease-inter­vention activities on the lives of their clients (Rothenberg and Paskey 1995). Members of the community, including those at high, low and no risk of STIs, must also be aware and supportive of the important work being done to prevent the spread of HIV and other infectious agents. Public health problems will persist and public policies will continue to be misguided as long as they are based on prejudice, discrimination and ignorance (U.S. Congress 1995).

William W. Darrow served the Centers for Disease Control and Prevention as a Research Sociologist for 33 years. He is now a Professor of Public Health and Sociology at Florida International University. His current interests include community organization for health promotion, program planning and the evaluation of intervention designed to prevent HIV transmissions.

John J. Potterat has specialized in the epidemiology of sexually transmissible infections since 1968 and has directed STD/HIV control efforts at the health department in Colorado Springs since 1972. During the last decade he has focused on the collection of empirical data to elucidate the influence of network structure on infectious disease propagation.

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Richard B. Rothenberg is currently Professor in the Department of Family and Preventive Medicine at the Emory University School of Medicine. He works on the transmission dynamics of HIV/AIDS and STDs, with an interest in the role of social networks in disease propagation.

Donald E. Woodhouse worked with the Colorado Springs HIV/STD Control Program until 1994. He presently coordinate grants and sponsored research at Lock Haven University of Pennsylvania and develops health outreach programs in rural communities.

Stephen Q. Muth has managed computer assisted analysis for the STD/HIV Programs at the health department in Colorado Springe since 1988. Current efforts include merging spatial analysis techniques using GIS with those of social networks analysis.

Alden S. Klovdahl developed an interest in social networks and infectious diseases in the late 1970s. Data on the first "cluster" of AIDS patients facilitated the development of his ideas. He is currently looking at the spread of sexually transmitted diseases through networks (in Baltimore) and tuberculosis (in Houston).

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