ABSTRACT: Bridge populations can play a central role in the spread of human immunodeficiency virus (HIV) by providing transmission links between higher and lower prevalence populations. While social network methods are well suited to the study of bridge populations, analyses tend to focus on dyads (i.e., risk between drug and/or sex partners) and ignore bridges between distinct subpopulations. This study takes initial steps toward moving the analysis of sexual network linkages beyond individual and risk group levels to a community level in which Chicago's 77 community areas are examined as subpopulations for the purpose of identifying potential bridging communities. Of particular interest are "hidden" bridging communities; that is, areas with above-average levels of sexual ties with other areas but whose below-average AIDS prevalence may hide their potential importance for HIV prevention. Data for this analysis came from the first wave of recruiting at the Chicago Sexual Acquisition and Transmission of HIV Cooperative Agreement Program site. Between August 2005 through October 2006, respondent-driven sampling was used to recruit users of heroin, cocaine, or methamphetamine, men who have sex with men regardless of drug use, the sex partners of these two groups, and sex partners of the sex partners. In this cross-sectional study of the sexual transmission of HIV, participants completed a network-focused computer-assisted self-administered interview, which included questions about the geographic locations of sexual contacts with up to six recent partners. Bridging scores for each area were determined using a matrix representing Chicago's 77 community areas and were assessed using two measures: non-redundant ties and flow betweenness. Bridging measures and acquired immunodeficiency syndrome (AIDS) case prevalence rates were plotted for each community area on charts representing four conditions: below-average bridging and AIDS prevalence, below-average bridging and above-average AIDS prevalence, above-average bridging and AIDS prevalence, and above-average bridging and below-average AIDS prevalence (hidden bridgers). The majority of the 1,068 study participants were male (63%), African American (74%), and very poor, and the median age was 44 years. Most (85%) were sexually active, and 725 provided useable geographic information regarding 1,420 sexual partnerships that involved 57 Chicago community areas. Eight community areas met or came close to meeting the definition of hidden bridgers. Six areas were near the city's periphery, and all eight areas likely had high inflows or outflows of low-income persons displaced by gentrification. The results suggest that further research on this method is warranted, and we propose a means for public health officials in other cities to duplicate the analysis.
