San Francisco Department of Public Health, San Francisco, California, USA.
Received 26 July, 2007
Accepted 15 August, 2007
In a community-based survey in San Francisco, black men who have sex with men (MSM) had higher rates of same-race/ethnicity sexual partnerships and partners 10 or more years older compared with other MSM. Differences in sexual networks may explain why black MSM have higher HIV prevalence than other MSM despite lower levels of risk behavior.
Higher HIV prevalence despite lower reported risk behavior among black men who have sex with men (MSM) compared with other MSM remains an unexplained paradox . Millett et al.  offered 12 hypotheses that might explain the discrepancy, six of which they considered to have insufficient or conflicting evidence. One such hypothesis, that the sexual networks of black MSM place them at greater risk of HIV than the sexual networks of other MSM, has been difficult to examine in large community-based studies because it requires detailed, partner-by-partner information.
We analysed data from the National HIV Behavioral Surveillance conducted in 2004 in San Francisco (n = 1574) to examine interracial and intergenerational partnering of MSM. The methods and primary results of the National HIV Behavioral Surveillance on a national level have been described in detail previously . Briefly, the survey is based on time–location sampling at venues frequented by MSM . The national survey collected demographic information, substance use, and sexual risk behaviors on an individual level. The local questionnaire in San Francisco collected partner-by-partner information, including partner race/ethnicity, age, and HIV serostatus, and sexual behavior within each partnership. HIV serological testing was conducted on a consecutive subsample (n = 450 in San Francisco).
Of MSM enrolled, 108 (6.9%) were black, 191 (12.2%) were Asian/Pacific Islander (API), 293 (18.7%) were Latino, 859 (54.8%) were white, and 123 (7.8%) were other/unknown. HIV prevalence was highest among black MSM by self-report (22.6% versus 6.2% among API, 13.7% among Latino, and 18.8% among white individuals), and by serological testing in the subsample (31.8% among black, versus 10.0% among API, 23.1% among Latino, and 26.3% among white individuals), although neither achieved statistical significance (P = 0.07 and 0.14, respectively; tests are chi-square unless otherwise stated). Compared with white MSM, black MSM reported fewer partners in the past year (mean 10 versus 20, Kruskall–Wallis test P = 0.002), less unprotected anal intercourse (UAI) in the past 6 months (22.8 versus 34.9%, P < 0.001), and less UAI within potentially HIV-serodiscordant partnerships (6.0 versus 11.9%, P < 0.001). Injection drug use, a history of sexually transmitted infection in the past year, and ever testing for HIV did not differ by race/ethnicity.
The 1574 participants described 3730 partnerships, including 264 of black MSM, 383 of API, 703 of Latino, and 2069 of white individuals. Using the distribution of race/ethnicity in our sample, we calculated an expected number of same race/ethnicity partnerships that would occur if there were no selection by race/ethnicity (i.e. that partnerships form randomly with respect to race/ethnicity; Table 1). The expected number of same race/ethnicity partnerships was compared with the actual observed number using the chi-square test. Black MSM had a 3.2 ratio of observed-to-expected same race/ethnicity partnerships (P < 0.001); the ratio was 1.5 for API (P = 0.08), 1.4 for Latino (P = 0.07), and 1.1 for white (P = 1.0) individuals. Compared with white MSM, black MSM were more likely to have a partner 10 or more years older (10.3 versus 5.9%, P = 0.03). API MSM were more likely to have a partner within 10 years of their own age compared with white MSM (81.1 versus 67.6%, P < 0.001).
Our findings echo other studies showing higher HIV prevalence but the same or lower levels of risk behavior among black MSM compared with other MSM. We also confirmed the findings of one previous study from Los Angeles that black MSM have a significant tendency towards having partners of their own race/ethnicity and for older partners . The combination of interracial and intergenerational sexual mixing may explain why the prevalence of HIV initially became higher among black MSM, and why the high prevalence has been sustained into the third decade of the epidemic. In a relatively small minority population, same-race/ethnicity partnering may create closely interconnected sexual networks, such that once HIV enters the network, it spreads quickly through it. Same race/ethnicity partnering alone, however, may not be sufficient, as evidenced by API MSM, who have the second highest same race/ethnicity partnering but the lowest HIV prevalence. Intergenerational sex has been proposed to be responsible for sustained high HIV prevalence as a result of the continual transmission from older aged cohorts (in whom there is a high cumulative prevalence of HIV) to younger aged cohorts (most of whom are susceptible to infection) in MSM in the United States and in the general population of sub-Saharan Africa [5,6]. Our observations among API MSM further support this hypothesis; API MSM had the strongest tendency for partners within their own age group and continue to have the lowest HIV prevalence into the third decade of the epidemic, despite equally high or higher levels of individual risk behavior [7–9]. That is, same race/ethnicity partnering is risky to the network when there are large age gaps between partners, but protective to the network when age gaps are small.
Intervening on the network level will be extremely challenging, as people's sexual preferences by age and race/ethnicity may not be feasible or desirable to change for many. Further investigation is needed to determine whether partnering selections are the result of personal preferences or structural factors, such as economic and geographical segregation by race. New interventions focusing on networks will be difficult to develop and evaluate in randomized controlled trials. In the meantime, individual-level interventions with confirmed efficacy will need to be improved and intensified in order to achieve the same benefit for black MSM.
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