Two-thirds of partnerships had geographical partner data (n = 510) and 273 participants (89%) provided geographical data on at least 1 partner. Almost half of these partnerships (238, or 47%) were spatially assortative with 69% of these within the same census tract. Almost two-thirds (65%) of the 273 participants providing partner data reported at least 1 spatially assortative partnership, and 40% of participants reported that all partnerships were spatially assortative.
Spatial, racial, and age assortativity were calculated by gender and partnership type (Table 3). Spatial assortativity was most common in women's exchange partnerships (74%) but less common among all types of men's partnerships. Partnerships in this population were highly assortative by race. Approximately half of partnerships were assortative by age within 5 years.
Participants residing in HIV core areas were significantly more likely to have spatially assortative partnerships than participants from the noncore areas (Table 4). Participants reporting 1 or 2 sexual partners were significantly more likely to have spatially assortative partnerships compared to participants reporting 3 or more partners. Exchange partnerships were significantly more likely to be spatially assortative than main or casual partnerships, though this differed by gender. Spatial assortativity was significantly more common among women's exchange partnerships than men's exchange partnerships (74% and 43%, respectively, P = 0.01). Unprotected intercourse was marginally more likely in spatially assortative partnerships (P = 0.09).
After accounting for the correlation between multiple partnerships per participant with generalized estimating equation, and adjusting for partnership type, number of partners, and unprotected intercourse, participants from HIV core areas were marginally more likely to have spatially assortative partnerships than participants from noncore areas (PR: 1.2, 95% CI: 1.0–1.4). Unprotected intercourse was significantly more likely among spatially assortative partnerships. Concurrency, racial assortativity, and age assortativity were not significantly associated with spatial assortativity. When we considered spatial assortativity as partners residing in the same census tract, we found that HIV core area residence remained associated with spatial assortativity (PR: 1.3, 95% CI: 1.0–1.7), though unprotected intercourse and exchange sex were no longer strongly associated with spatial assortativity.
Partners' census tracts were missing for 34% of the 776 partnerships eligible for this analysis. Several factors were associated with missing partner data. Exchange partnerships were the most likely to have missing data, followed by casual and main partnerships (51%, 42%, and 19%, respectively). When participants met their partners in their neighborhoods, only 24% of partner census tracts were missing, compared to 46% when they did not meet in the neighborhood. Less recent partnerships were more likely to have missing data than most recent partnerships. We imputed the missing assortativity data using 3 variables identified above, plus gender. HIV core area residence remained associated with spatial assortativity (PR: 1.2, 95% CI: 1.0–1.5), though unprotected intercourse was no longer strongly associated with spatial assortativity.
In a population recruited through venue-based sampling, almost half reported choosing spatially assortative partners. Participants who lived in the HIV core areas were more likely to choose spatially assortative partners than residents of non core areas after adjusting for partnership type, gender, and number of partners. This relationship persisted in sensitivity analyses. Women who engaged in exchange sex were most likely to report spatial assortativity. Our study confirms Zenilman et al previous work, which was limited to STD clinic patients.13 This remained the case even when sampling from a non-STD clinic population and using census tract data rather than geocodable address.
Choosing a sex partner from a high HIV prevalence area puts an individual at high risk of selecting an infected partner, with exchange sex and unprotected sex increasing the probability of HIV acquisition even further. Spatial assortativity was common but not universal in this population, indicating that partners' residential exposures should be measured when studying the effect of neighborhood-level factors on sexual behavior.
Contextual factors that may impact geography of partner selection include access to public transportation, segregation of residential housing, natural boundaries like rivers and roads, and access to specific social institutions that create opportunities for meeting.31 Some of the differences in spatially assortative partnering between the HIV core and noncore areas could reflect differential access to transportation, with car ownership allowing more latitude in partner selection. Although we did not ask about transportation access, Census data indicate that household car ownership in the HIV core census tracts was significantly lower than in the noncore census tracts (45% vs. 69%, P < 0.0001).27 Although differences in car ownership may be related to poverty, residence in high poverty areas does not confound the observed relationship between HIV core area residence and spatially assortative partnerships in this analysis. Residents living in census tracts in the top quartile of poverty in Baltimore City were no more likely than the residents of other census tracts to report spatially assortative partnerships (52% vs. 48%, P = 0.14).
Racial assortativity was common in this population, with black women consistently displaying the most highly assortative partner selection.25,32–38 The near universality of racial assortativity in all age groups and partnership types indicates that it should be taken into account in studies of partner selection. We found that this population was moderately assortative by age, somewhat lower than the 75% to 83% assortativity observed by Laumann et al among men and women and 75% assortativity calculated by Darroch et al among women nationwide.25,39 Age assortativity may not play a large role in HIV/STD risk among adults; whereas disassortative mixing by age increases risks of STD among adolescent girls,7,39–44 the same association is not seen among adult women.7,43,45
This study is subject to several limitations, including missing data and imprecise measurement. We collected data on only the 5 most recent sexual partners. Of the 246 participants who knew where their partners lived, 11% had more than 5 partners. A large proportion of these (46%) had 6 to 7 partners. Given that participants with more partners are less likely to have spatially assortative partnerships (Table 4) and that spatial assortativity is less likely for less recent partners (data not shown), we expect that partnerships 6 and 7 would not be spatially assortative. Most (69%) participants with 6 to 7 partners lived outside of HIV core areas. These missing partnerships may have caused us to underestimate the true association between HIV core area and spatial assortativity.
Also of note is the proportion of participants who were unable or unwilling to report where their sex partners lived. This may be due to not knowing where their partners lived (particularly in the case of casual and exchange partnerships) or not remembering where sex partners lived (particularly in the case of less recent partners or greater number of recent partners). Explicit refusals constituted 1% of missing data on partnerships, with the remainder presumably from participants who did not know their partners' residences. Although the high proportion of missing data are consistent with other estimates in the literature on missing partner data in partner notification,46,47 the missing outcome data increases the uncertainty in our estimates. We were, however, able to assess the impact of the missing data in our regression models using multiple imputation and other sensitivity analyses. The sensitivity analyses generally confirmed the relationship of HIV core areas and spatially assortative partnerships, though associations with other factors (e.g., unprotected intercourse, exchange sex) were less consistent. Given previous work on the biases because of missing data in sexual network analysis, further research on missing spatial data in the analysis of partner selection data are needed.48
Without geocodable address, we could not calculate Euclidian distances between partners to check consistency of our results with the previous literature.13,15 Study protocols required the use of census tract rather than census block group to protect participant confidentiality. Self-reported data on sensitive subjects may be subject to social desirability bias. Although participants may have felt that they should not report risk behaviors like concurrent partnerships, the prevalence of these behaviors was so high that this bias is unlikely.
Our study used a venue-based sampling strategy to locate the general population at high risk for HIV/STD, increasing its generalizability. We were able to examine the relationships between several types of assortativity in a high-risk population, including demographic and spatial assortativity. We were able to link spatial proximity, and reported risk behaviors, and found that residence in an HIV core area was independently associated with increased likelihood of spatial assortativity. This study therefore has implications for future public health practice and research on geographical and contextual factors in HIV/STD prevention.
Evidence that high HIV prevalence areas also have geographically denser sexual networks could help health departments decide to continue targeting screening to these core areas, rather than taking a generalized screening approach. Prevention programs could also capitalize on the spatial proximity of partners in high-prevalence areas by supplementing their individual-focused prevention messages with social marketing campaigns (e.g., billboards, leaflets).49
Shared geographical space may be the underlying mechanism by which social norms are developed in a network.50 New studies assessing the impact of social norms and attitudes on sexual behavior should take geographical context into account. Structural influences on local sex partner availability (i.e., because of high levels of incarceration) may strongly impact partner selection patterns (i.e., concurrency) in areas where spatial assortativity is common.51 We would expect to see particularly strong associations between contextual factors and behavior when both partners are influenced by the same environment.
These data were collected in the Baltimore City-specific questionnaire of the National HIV Behavioral Surveillance system. Future waves of this national survey should add items on partner's residence in regards to comparing spatial assortativity across regions and populations at high risk for HIV/STD infection. In 1999, Zenilman et al recommended targeted neighborhood approaches to screening and intervention in urban core areas. It appears that these recommendations are still warranted.
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