Authors of a comprehensive review of literature reporting data on per-act HIV transmission risks have updated the last estimates produced by the U.S. Centers for Disease Control and Prevention in 2005, and in the process have highlighted the importance of treatment and prevention reaching those at greatest risk.
The CDC’s estimates “relied heavily on estimates derived from a single study of heterosexual couples,” according to the review’s authors. Since then, studies have yielded new data on transmission risks among men who have sex with men as well as heterosexuals. In addition, studies have produced data on the effects of “modifying factors” on transmission probability, including genital ulcers, circumcision, condom use, pre-exposure prophylactic use of antiretroviral drugs, viral load, and antiretroviral treatment for infected individuals. The new estimates, published in AIDS, the official journal of the International AIDS Society, were based on data from studies examining per-act HIV 1 transmission probabilities, and studies examining factors that modify risk. Authors of the review, Estimating per-act HIV transmission risk, looked at abstracts from more than 7,000 articles pertaining to transmission risks, honed that down to 14 articles detailing per-act HIV transmission estimates and narrowed more than 8,000 articles looking at modifying factors down to 15.
Authors estimate that the per-sexual act HIV transmission risk varies from four infections for every 10,000 exposures through penile-vaginal intercourse to 138 for every 10,000 exposures through receptive anal intercourse. While the effort highlighted data from developed regions that most closely reflect the HIV epidemic in the United States, insights that it yielded appear pertinent to program planning, particularly to reach men who have sex with men and others whose ability to access treatment and other prevention measures is limited, in other regions as well. Estimates of transmission risk from receptive anal intercourse rose from previous estimates. Risks for all sexual exposures were reduced most by combined use of condoms and antiretroviral treatment of infected partners.