August 12, 2005
In the current study, the authors examined the relationship between social distance (measured as the geodesic, or shortest distance, between two people in a connected network) and geographic distance (the actual distance between them in kilometers [km]) and the effects of each on the potential transmission of HIV. The investigators used data from a longitudinal cohort study of 595 people at risk for HIV and their sexual and drug-using partners (total N=8,920 unique individuals) conducted in Colorado Springs, Colo., from 1988 to 1992, in which sociodemographic, clinical, behavioral and network information about participants was ascertained. The researchers used place of residence as the geographic marker and calculated the distance between people grouped by various characteristics of interest.
The study found that a distance of 4 km or less separated 52 percent of all dyads. Pairs closest to each other were those who both shared needles and had sexual contact (mean=3.2 km) and HIV-positive people and their contacts (mean=2.9). Prostitutes and their paying partners were the most distant pairs (mean=6.1 km).
In a connected subset of 348 participants, nearly half the people were between 3-6 steps from each other in the social network and were separated by a geographical distance of 2-8 km. "Using block group centroids, the mean distance between all persons in Colorado Springs was 12.4 km compared with a mean distance of 5.4 km between all dyads in this study (P<0.0001)," the authors wrote. "The subgroup of HIV-positive people and their contacts was drawn in real space on a map of Colorado Springs and revealed tight clustering of this group in the downtown area."
"The association of social and geographic distance in an urban group of people at risk for HIV provides demonstration of the importance of geographic clustering in the potential transmission of HIV," the researchers concluded. "The proximity of persons connected within a network, but not necessarily known to each other, suggests that a high probability of partner selection from within the group may be an important factor in maintenance of HIV endemicity."