John J. Potterat

Social networks and infectious disease: The Colorado Springs study

The social network paradigm provides a set of concepts and methods useful for studying the structure of a population through which infectious agents transmitted during close personal contact spread, and an opportunity to develop improved disease control programs. The research discussed was a first attempt to use a social network approach to better understand factors affecting the transmission of a variety of pathogens, including hepatitis B virus (HBV) and human immunodeficiency viruses (HIV), in a population of prostitutes, injecting drug users (IDU) and their personal associates in a moderate-sized city (Colorado Springs, CO). Some of the challenges of studying large social networks in epidemiological research are described, some initial results reported and a new view of interconnections in an at risk population provided. Overall, for the first time in epidemiologic research a large number of individuals (over 600) were found connected to each other, directly or indirectly, using a network design. The average distance (along observed social relationships) between persons infected with HIV and susceptible persons was about three steps (3.1) in the core network region. All susceptibles in the core were within seven steps of HIV infection.

Social network dynamics and HIV transmission.

Objective:To prospectively study changes in the social networks of persons at presumably high risk for HIV in a community with low prevalence and little endogenous transmission. Methods:From a cohort of 595 persons at high risk (prostitutes, injecting drug users, and sexual partners of these persons) and nearly 6000 identified contacts, we examined the social networks of a subset of 96 persons who were interviewed once per year for 3 years. We assessed their network configuration, network stability, and changes in risk configuration and risk behavior using epidemiologic and social network analysis, and visualization techniques. Results:Some significant decrease in personal risk-taking was documented during the course of the study, particularly with regard to needle-sharing. The size and number of connected components (groups that are completely connected) declined. Microstructures (small subgroups of persons that interact intensely) were either not present, or declined appreciably during the period of observation. Conclusions:In this area of low prevalence, the lack of endogenous transmission of HIV may be related in part to the lack of a network structure that fosters active propagation, despite the continued presence of risky behaviors. Although the relative contribution of network structure and personal behavior cannot be ascertained from these data, the study suggests an important role for network configuration in the transmission dynamics of HIV.

Gonorrhea as a social disease

Gonococcal infection in Colorado Springs, Colorado, is concentrated in about 1% of the population. The social groups at risk are characterized as young, nonwhite, heterosexual, and connected to the military. They exhibit residential proximity by clustering in “core” census tracts; 51% of cases were in four tracts. They domonstrate residential stability and close social association at preferred sites for nighttime leisure activity (six major site out of 300 available). Social aggregation is further domonstrated by the length of social contract prior to sexual contact (45% had known each other for over two months). the neighborhood nature of sexual choices, and the grouping of sexually connected individuals in lots (six lots contained 20% of cases). The force of infectivity, measured in person-days of potential spread of gonorrhea by infected contacts, provides a quantitative assessment of the importance of identifiable social groups in the transmission of gonorrhea.

Network structural dynamics and infectious disease propagation

We aimed to relate dynamic changes in risk-network (sex and/or injecting drug) structure to observe STD/HIV transmission. We analysed macroand micro-structural elements in 2 heterosexual networks, augmented by ethnographic observations. In a Colorado cohort of injecting drug users (n=595), measures of subgroup formation and of density of activity show decrease of network cohesion over time; only one HIV transmission was observed in 3 years. In a group of adolescent heterosexuals in Georgia (n=99), the reverse process (increase in structural cohesion) was associated with efficient syphilis transmission: 10 cases were observed. Changes in personal risk behaviours over time were modest. STD/HIV transmission patterns were associated with intensification or diminution of network cohesion. Network and ethnographic data suggest that enhanced connectivity facilitates transmission while segmentation impedes it, suggesting opportunities for interventions. These data also emphasize the need to re-evaluate purely behavioural explanations of STD/HIV transmission.

Mapping a social network of heterosexuals at high risk for HIV infection

Objective:To determine how heterosexuals at risk for HIV infection interconnect in social networks and how such relationships affect HIV transmission. Design:Cross-sectional study with face-to-face interviews to ascertain sociosexual connections; serologic testing. Participants:Prostitute women (n=133), their paying (n=129) and non-paying (n=47) male partners; injecting drug users (n= 200) and their sex partners (n=41). Participants were recruited in sexually transmitted disease and methadone clinics, an HIV-testing site, and through street outreach in Colorado Springs, Colorado, USA. Main outcome measures:Reported behaviors, risk perceptions, sociosexual linkages, and HIV prevalence. Results:Respondents were well informed, but reported engaging in high-risk behaviors frequently. Nevertheless, over 70% of respondents perceived themselves to be at low risk for HIV infection. The 595 respondents identified a social network of 5162 people to which they belonged. Network analytic methods indicated 147 separate connected components of this network; eight of the 19 HIV-positive individuals in the network were located in smaller components remote from the largest connected component. Conclusion:The isolated position of HIV-positive individuals may serve as a barrier to HIV transmission and may account for the lack of diffusion of HIV in heterosexual populations in this region. Network analysis appears useful for understanding the dynamics of disease transmission and warrants further development as a tool for intervention and control.

Partner notification for HIV and STD in the United States: low coverage for gonorrhea, chlamydial infection, and HIV.

Background Little is known about the scope of current public health partner-notification (PN) activities in the United States. Goal The goal of the study was to define what PN services U.S. health departments provide in areas with high STD/HIV-related morbidity. Study Design The study involved a survey of STD program staff members in U.S. areas with the highest reported rates of infectious syphilis, gonorrhea, chlamydia, and HIV in 1998. Results Staff members of 60 (77%) of 78 health departments provided data. PN interviews were conducted with 7583 (89%) of 8492 cases of syphilis, 23,097 (17%) of 139,287 cases of gonorrhea, and 26,487 (12%) of 228,210 cases of chlamydia. In areas with mandatory HIV reporting, 4375 (52%) of 8328 persons infected with HIV were interviewed for PN. Conclusions Except for patients with syphilis, public health PN services affect only a minority of persons with STD or HIV infection in high-morbidity areas of the United States.

HIV infections in sub-Saharan Africa not explained by sexual or vertical transmission

An expanding body of evidence challenges the conventional hypothesis that sexual transmission is responsible for more than 90% of adult HIV infections in Africa. Differences in epidemic trajectories across Africa do not correspond to differences in sexual behaviour. Studies among African couples find low rates of heterosexual transmission, as in developed countries. Many studies report HIV infections in African adults with no sexual exposure to HIV and in children with HIV-negative mothers. Unexplained high rates of HIV incidence have been observed in African women during antenatal and postpartum periods. Many studies show 20%–40% of HIV infections in African adults associated with injections (though direction of causation is unknown). These and other findings that challenge the conventional hypothesis point to the possibility that HIV transmission through unsafe medical care may be an important factor in Africas HIV epidemic. More research is warranted to clarify risks for HIV transmission through health care.

Pathways to prostitution: The chronology of sexual and drug abuse milestones

To assess the sequence, timing, and prevalence of sexual and illegal drug use milestones in prostitute women, we interviewed 237 prostitutes in the community and 407 comparison women at an STD clinic. Drug use was more commonly reported by prostitutes than comparisons (86% vs. 23%), as was non‐consensual prepubertal sex (32% vs. 13%). Sexual‐ and drug‐related milestones occurred in the same order in both groups, with drug use preceding sexual activity and injecting drug use preceding prostitution. Ninety‐four percent of prostitutes who injected drugs reported noninjectable drug use before prostitution, and 75% of prostitutes who injected drugs reported doing so before beginning prostitution. The age distributions at critical events were similar for prostitutes and comparison women who reported regular drug use. Comparison women who did not report regular drug use were in general older than both these groups at the time of early sexual experience and drug experimentation. However, the ordering of these eve...

EPIDEMIOLOGIC DIFFERENCES BETWEEN CHLAMYDIA AND GONORRHEA

To assess the prevalence, demographics, and transmission patterns of genital chlamydia infection, we screened 3,078 patients, and compared identified cases (N = 511) to gonorrhea cases (N = 291) diagnosed in the same setting. Chlamydia cases were younger and more likely to be White than their gonorrhea counterparts. Chlamydia cases were distributed diffusely; geographic overlap between the two diseases was only about 40 percent. Gonococcal coinfection was noted in less than 10 percent of patients with chlamydia. Nearly half of men with chlamydia and four-fifths of women were asymptomatic and most cases were identified through screening or contact tracing. Populations at high risk for chlamydia are seemingly different from those for gonorrhea. Differences may be due to control interventions (active for gonorrhea, passive for chlamydia). Chlamydia case reporting and control initiatives are recommended.

Let it be sexual: how health care transmission of AIDS in Africa was ignored

The consensus among influential AIDS experts that heterosexual transmission accounts for 90% of HIV infections in African adults emerged no later than 1988. We examine evidence available through 1988, including risk measures associating HIV with sexual behaviour, health care, and socioeconomic variables, HIV in children, and risks for HIV in prostitutes and STD patients. Evidence permits the interpretation that health care exposures caused more HIV than sexual transmission. In general population studies, crude risk measures associate more than half of HIV infections in adults with health care exposures. Early studies did not resolve questions about direction of causation (between injections and HIV) and confound (between injections and STD). Preconceptions about African sexuality and a desire to maintain public trust in health care may have encouraged discounting of evidence. We urge renewed, evidence-based, investigations into the proportion of African HIV from non-sexual exposures.