July 27, 2012
High Viral Loads Linked to HIV Risk Among Black MSM
by Laura Whitehorn
The high HIV incidence among black men who have sex with men (MSM) in the United States may be explained, at least in part, by the finding that HIV-negative black MSM, compared with white MSM, are more than twice as likely to encounter a sex partner who can transmit HIV if safer sex practices are not followed. Eli Rosenberg, PhD, a researcher at the Emory University Rollins School of Public Health, delivered that startling conclusion on Monday, July 23, at the XIX International AIDS Conference (AIDS2012) in Washington, DC.
Rosenberg’s report is based on data from the InvolveMENt study, an ongoing cohort of black and white MSM in Atlanta.
The incidence and prevalence of HIV among black MSM, particularly young black MSM, continue to be disproportionately high compared with those among MSM of other races or ethnicities. Yet in studies, black MSM consistently report the same or lower levels of sexual- or drug-using risk behaviors as other MSM.
Key data from other studies presented at the conference, including a CDC analysis of approximately 200 studies and HIV Prevention Trials Network study 061, suggest that structural disparities—including the fact that HIV-positive black MSM are less likely to know they’re positive or to be successfully engaged in care and treatment, thus they’re potentially more infectious than other MSM—are key drivers of the epidemic in black MSM communities.
“To acquire HIV, it is necessary to encounter somebody who is not only positive but has unsuppressed virus capable of transmission,” Rosenberg explained.
To explore this particular disparity, Rosenberg and his InvolveMENt peers had to develop new measurements. “HIV prevalence—the total number of HIV-positive people over the total population—provides the probability of encountering someone with the virus but doesn’t incorporate viral load,” Rosenberg explained. On the other hand, he said, the standard community VL measurement counts all those who have been diagnosed HIV-positive, but not undiagnosed cases. “We’re stuck with a limited set of measures,” he said, “because community viral load is only among those diagnosed with HIV.”
The researchers therefore created a tool—transmission potential prevalence, or TPP—to calculate how many black and white MSM participating in the study were both HIV positive and had viral loads high enough to pass it on to others. They applied the tool to the 709 participants who have enrolled in InvolveMENt, which Rosenberg described as an HIV incidence cohort study of sexually active black and white MSM in Atlanta between the ages of 18 and 40.
The black men were on average younger, with a median age of 25 compared with 28 for the white men.
Reported rates of unprotected anal intercourse were consistent between black and white men in the cohort—an average of two instances within the year leading up to study participation for both groups. Racial mixing patterns for partner selection were 70 percent exclusive, meaning that most MSM in the study chose sex partners from their own racial group.
HIV prevalence was 42 percent in the black group compared with 14 percent among the white MSM. Incidence (new HIV cases) in the study was found to be 6.4 cases per 100 person-years among those in the black MSM group, compared with 1 case per 100 person-years in the white MSM group. “In other words,” Rosenberg explained, “if we had 100 HIV-negative black MSM, one year later we would see six cases develop, while among 100 white [MSM] we would see one case develop over the course of a year.”
Viral load data were similar between the black and white groups if measured in men previously diagnosed with HIV, he said. But the InvolveMENt study was able to calculate previously undiagnosed cases, because upon entering the study, participants were asked whether they had HIV, then tested for the virus. That allowed researchers to create a category of previously undiagnosed HIV-positive men and to use that to measure cases that would otherwise be counted as HIV negative.
By expanding the parameters to include everyone—undiagnosed as well as diagnosed—the researchers found 25 percent of the black MSM were both HIV positive and had viral loads above 400, as compared with 8 percent among the white participants.
Translating these data into the probability of encountering a sex partner who is capable of transmitting HIV, the study found that black MSM faced a 39 percent chance that at least one partner has transmission potential, compared to 18 percent among white men at the same risk behavior level.
Rosenberg explained what this means: “To have a 50 percent chance of acquiring HIV, a black MSM needs just three partners, compared to seven for white MSM. To reach a 90 percent risk of HIV, a black man would need 10 partners, compared to 25 for white MSM.”
The study has limitations, he said, including that other possible causes of higher incidence, such as the presence of other sexually transmitted infections and sero-sorting (choosing partners who have the same HIV status).
The researchers conclude that HIV prevention approaches that rely solely on changing risk behavior will not work to eliminate racial disparities. They recommend that new resources aimed at preventing transmission be focused on communities with a high TPP, and that viral load measurements be incorporated into HIV surveillance measures in a manner that accounts for TPP. This, Rosenberg said, “will help us to understand transmission risk in a meaningful way.”
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