Treatment and sexual transmission risk between men

Published: January 21, 2014

 Although antiretroviral treatment is highly likely to provide some preventive benefit for gay men and other men who have sex with men, it is currently unknown whether it will reduce HIV infectiousness via anal sex to the same extent as via vaginal sex. Although there were 38 male/male couples (3%) in HPTN 052, there were not enough data to analyse sexual transmission risk between men separately.

 
A US study conducted prior to the widespread availability of potent antiretroviral therapy found higher rates of HIV transmission during anal sex than vaginal sex. The higher rate is thought to be due primarily to biological differences between the vagina and the anus.1
 
However, a more recent Australian study found that gay men’s risk of acquiring HIV remains similar to the pre- treatment era.2 These findings remain unconfirmed by other researchers or fully explained.
 
Studies suggest that receptive anal sex may be 8-times (if the HIV-positive male does not ejaculate) and 18-times (with ejaculation) more risky than receptive vaginal sex. Insertive anal sex appears to be 2.75-times (if the HIV-negative male is circumcised) and 7.75-times (uncircumcised) more risky than insertive vaginal sex.3 
 
In addition to the increased risks of anal sex compared to vaginal sex, there are not enough data to reliably state that reductions of viral load in the blood are paralleled in rectal secretions. Paired blood and rectal biopsy samples tested for antiretroviral resistance have shown different mutation profiles in the virus recovered from each site, suggesting that viral replication can persist in the rectum even if there is viral suppression in the blood and semen.4 
 
In a 2012 review examining the evidence for treatment’s impact upon the risk of sexual transmission between men, Muessig and colleagues concluded: “While the results of HPTN 052 demonstrated the capacity of ARVs to markedly reduce the risk of penile-vaginal transmission despite similar biological and pharmacokinetic uncertainties, we cannot be certain that this will be the case for anal intercourse given the much higher transmission probability in the absence of ART.”5
 
In other words, although treatment is likely to reduce the chance of infection between men (and, between heterosexuals who have unprotected anal sex) to the same degree as it does for vaginal sex, due to the higher HIV transmission risk of anal sex it is likely to require a more profoundly suppressed viral load to reduce the risk to the same extent as we have seen in heterosexual transmission studies.
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