- Category: HIV Prevention
- Published on Monday, 11 March 2013 00:00
- Written by Gus Cairns
An analysis of 4 studies of sexual risk and HIV infection in U.S. gay men, presented at the 20th Conference on Retroviruses and Opportunistic Infections (CROI 2013) last week in Atlanta, has found a 22% higher risk of HIV infection per sexual contact among black gay men that is not explained by other factors such as number of sexual partners, injecting drug use, or age.
Young men who have sex with men (MSM) and MSM of color have the highest HIV incidence rates in the U.S. Black people make up 12% of the U.S. population, but have 45% of new HIV diagnoses and, while new HIV diagnoses stayed steady among most population groups in the last 3 years, they increased by 48% in young black gay men.
This is not explained by individual risk behavior: young gay black men actually have fewer partners and lower rates of recreational drug use than other gay men, according to previous research.
Researchers from the University of California at San Francisco, and the San Francisco Department of Public Health used data from surveys of gay men that were conducted from 1992 to 2003 to find out if young men and black men had a higher per-contact risk of HIV infection that was not explained by other behavioral or demographic factors.
They looked at 4 studies:
- Jumpstart was a CDC study of HIV incidence in high-risk HIV negative gay and bisexual men conducted in 1992 to 1994, before combination antiretroviral therapy (ART) was available.
- HIVNET VPS (Vaccine Preparedness Study) was a behavioral survey of gay men in preparation for possible HIV vaccine trials conducted in 1995 to 1997, as ART was becoming available.
- EXPLORE(described in this report) was a large behavior-change trial in gay men in the ART era, conducted in 1998 to 2003.
- VAX004(described in the same report) was the first Phase 3 trial of an HIV vaccine, also conducted in 1998 to 2003.
In total there were 10,760 gay men in these studies, who paid 42,395 trial visits every 6 months. In all, there were 584 HIV infections.
The analysis was restricted to visits in which participants reported at least 1 episode of unprotected receptive anal intercourse, unprotected insertive anal intercourse, protected receptive anal intercourse, unprotected sex with a partner of unknown HIV status, or unprotected sex with more than 1 HIV negative partner.
The researchers looked at how, in the 4 different trials, the number of sexual contacts varied with the risk of acquiring HIV. They then related these to the kind of sex (insertive, receptive protected, or receptive unprotected) men reported and whether the partner’s status was HIV positive, HIV negative, or unknown. They also asked about injecting drug use, though in the analysis this had no effect on HIV infections.
Using these figures, they were able to calculate the chance of becoming infected with HIV per sexual contact of a given type, and to what degree unprotected sex increased this risk.
The risk of HIV infection per sexual contact with a known HIV positive partner was:
- 1 in 137 contacts for unprotected receptive anal sex (0.73%);
- 1 in 455 contacts for unprotected insertive sex (0.22%);
- 1 in 1250 contacts for protected receptive sex (0.08%), all with partners known to have HIV.
Interestingly, one can infer from this an 89% prevention efficacy for condom use for receptive anal sex -- higher than that seen in another CDC study that compared reported condom use over a 6-month period to HIV infections over the same period.
These figures are for the 3 studies that took place after ART availability. Figures were little different in the Jumpstart study, with per-contact risks of 1 in 167, 1 in 714, and 1 in 2500 for unprotected receptive, unprotected insertive, and protected receptive sex respectively. These risks were not statistically different from those in the other studies.
Averaged across studies, the risk for unprotected receptive sex with partners of unknown HIV status was 1 per 204 contacts -- not much less risky than with known HIV positive partners. Risk varied between studies, however, with the HIV risk in VAX004 being 72% higher and in EXPLORE being 13% lower than in the VPS study.
Unadjusted for other factors, men under 25 had a 31% higher per-contact risk of being infected, and black men had a 78% higher risk. This was not because black men had more unprotected sexual contacts. On the contrary, they had considerably fewer -- just 3 for receptive sex and 5 for insertive sex on average in 6 months, compared with 11 and 12, respectively, for white men.
The average per-contact risk also varied with age: for unprotected receptive sex (with an HIV positive partner) it was 1 in 102 for under-25s, 1 in 115 for 25 to 30 year olds, and 1 in 156 for the over-30s.
It varied just as much, if not more, for race/ethnicity: 1 in 141 for white men (and almost the same for Latinos) and 1 in 96 for black men and "others" (which were mainly men of mixed race). However, because of relatively few trial visits and HIV infections among black men, this difference was not actually statistically significant. Adjusting for age reduced the increased risk to a 22% higher risk for black men (because they tended to be younger), but an added risk remained.
Why the higher risk for black men? This is the unanswered question, and isn’t explained by this study -- the findings of which have been observed in other studies too. Black men were a minority in the studies used, and they made fewer study visits, so the figures for them have more uncertainty.
It may be simply because HIV prevalence is higher among U.S. black men so they are generally in a higher-risk environment. Health inequalities also mean that partners are less likely to have undetectable viral loads.
Other factors include network effects, meaning that black men tend to have sex partners from a smaller pool of mainly black partners, and more age-mixing, meaning that black men in some studies have been found to be more likely to have partners who are significantly older than them -- and therefore more likely to have HIV.
Other factors may include different degrees of knowledge of partners’ HIV status, and presenter Hyman Scott commented that there was some evidence that black men might not so often be doing other things that reduce HIV risk, such as withdrawal before ejaculation.
H Scott, E Vittinghoff, and S Buchbinder. Age and Racial Disparities in Per-contact Risk of HIV Seroconversion among Men Who Have Sex with Men: US. 20th Conference on Retroviruses and Opportunistic Infections. Atlanta, March 3-6, 2013. Abstract 91.