This study demonstrates that selection of an older sex partner is significantly associated with PHI among a sample of young MSM in NC. After adjusting for covariates, we noted that having a sex partner 5 years older doubled the odds of PHI. Our findings expand upon and update the work of modelers of the MSM epidemic in the 1990s, before the advent of combination ARV therapy-in particular, 2 studies that used egocentric network analysis to assess the association between having an older sex partner and increased HIV risk. Both of these studies reflected the epidemic in the early 1990s, with a gradation of HIV prevalence directly proportional to the age of the men.
Morris et al investigated the effect of selecting older sex partners on the odds of HIV infection using data from the Longitudinal AIDS Impact Project in New York City.5 Seven waves of interviews were conducted with a closed cohort of MSM in New York City from 1985 to 1991. Comparing the partnerships of 71 young HIV-uninfected men with 10 HIV-infected counterparts, the results were striking. Among those participants who reported unprotected receptive anal sex, the seroprevalence was zero if all sex partners were less than 25 years old, and 44% among those with at least one sex partner over 25 years. For unprotected sex with an insertive partner, the prevalence leapt from zero when no partners were over 25 years old to 15% when at least one sex partner was older than 25 years. Service and Blower used empirical data collected in 1993 as part of a longitudinal study of HIV transmission among MSM in San Francisco to develop and test a predictive mathematical model estimating the likelihood of seroconversion within the cohort.6 Seropositive men had a greater probability of having more than one sex partner over age 30 years when compared with HIV-uninfected men-in both the 18-24 years age range (59% vs. 22%) and the 25-29 years range (42% vs. 70%). When the inputs to the model were changed from men having no sex partners over 30 years to all partners over 30 years, the seroprevalence jumped 4- to 5-fold. Thus, assortative age mixing seemed to be protective against HIV infection by limiting exposure to earlier “waves” of MSM with high HIV prevalence.8,9 Finally, Bingham et al analyzed Los Angeles' data from the Young Men's Survey in 1999-2000. Nearly 35% of MSM reported “mostly” having sex partners more than 5 years their senior-and their odds of HIV infection were 3.5 times greater than men with partners closer to their own age (95% CI: 1.6-7.8).7
Taken together, these studies and our work examine 3 distinct periods of the domestic HIV epidemic among MSM. Consider the Center's for Disease Control and Prevention's (CDC's) recent back-calculation estimates of HIV incidence, which showed the peak of infections among MSM in 1984-1985, a nadir in 1991-1993, and a steady increase since then to over 30,000 new cases estimated for 2003-2006.21 Morris et al initial data spanned 1985-1991; Service and Blower study sampled men in 1993; Bingham et al looked at the epidemic in 1999-2000; and our data are from 2007 to 2008. The late 1980s were characterized by a community-driven response to HIV among MSM. At least some of the decline in incidence can be attributed to the death of a substantial number of those infected. The early 1990s saw the impact of the first therapies, the height of the public health response, and the beginning of the reversal in HIV/AIDS-related mortality that has continued to the present. Despite the different prevailing approaches to HIV management in each period, age mixing represented a heightened risk for transmission. Is it possible that the potential impact of age mixing has simply been underestimated all of this time? Issues like sexual concurrency,22,23 Internet sex-seeking,24-26 and having sex while intoxicated26-28 are all associated with incident HIV infection, yet our screening methods remain almost exclusively focused on the traditional risk factors established at the start of the epidemic, like injection drug use or unprotected intercourse.29,30 Through behavioral surveillance among MSM with PHI and a better understanding of individual-level, “nontraditional” risks leading to their HIV infection, we have the potential to provide more tailored, contemporary prevention messages to high-risk populations. Addressing factors perceived as being less influential than traditional risks could, in aggregate, help to reduce the incidence of HIV. A compelling case can be made that age mixing, along with other “nontraditional” risk factors, ought to take a position alongside traditional factors at the forefront of behavioral surveillance.
Our most significant limitations center on the age of sex partners reported by participants. These men may not have accurately estimated the age of sex partners. Those with PHI might also introduce a differential recall bias regarding the age of sexual partners, having given more thought to risk behaviors following their HIV diagnosis. A single partner much older than the other 2 reported partners could skew the mean age upward, and would exaggerate the estimated effect of age on odds of PHI. However, when we examined the odds ratios in models using the oldest sex partner age and the mean age of partners, the point estimates were essentially unchanged (data not shown). The SNAP study was designed to determine the feasibility of using a respondent-driven sampling model to recruit and evaluate a cohort of MSM at high risk for HIV infection. This approach was an attractive option given its proven ability to sample at-risk hidden populations like illicit drug users,31,32 sex workers,33-35 and transgendered people.33,36 The extended “reach” of this design overcomes many of the issues of sampling bias that inherently limit studies using venue-based or facility-based sampling methods, thus improving external validity.37 However, because of poor recruitment from seeds living long distances from the study site, our controls may reflect a somewhat different base population than that from which the cases came. Because SNAP was intended to demonstrate our capacity to use participants as recruiters, we did not structure the study to include sufficient waves to reach network equilibrium (the point at which bias from nonrandom selection of seed participants is overcome).38
In summary, young MSM in NC who select older sex partners have significantly greater odds of acquiring HIV infection, even after controlling for specific high-risk behaviors. Our findings and earlier empirical data support the application of an individualized approach to counseling when talking to young MSM about their sexual risk behavior. Provision of safe sex messages should include both traditional and nontraditional risk factors, directed at all age groups of sexually active individuals. Young men who have older sexual partners should be informed of the comparatively increased risk that such partnerships pose for HIV infection. In parallel, older MSM living with HIV and engaged in care should receive secondary prevention messages encouraging disclosure of their status to partners, maintenance of safer sex behavior, and that ARV treatment alone is not enough to prevent transmission. Delivery methods like social marketing campaigns and new media could be important and novel ways of reaching a greater audience.
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