February 6, 2001
Just what is the risk of HIV infection for each act of vaginal intercourse for a man or for a woman whose partner is infected? What are the factors that increase transmission? What can be done to decrease the risk of transmission with each act of intercourse? (Makes it sound so romantic. . . ) The Rakai prevention trial provided an unusual chance to look at this in great detail. Of the 15,000 people in the initial trial, the investigators identified 415 discordant stable or married couples. Of those, there were 174 couples where both reported that they were exclusively monogamous. Using detailed information on the frequency of intercourse, the investigators estimated the risk of infection for each act of intercourse.
Overall, 18% of the men with infected partners became infected and 28% of the women with infected partners seroconverted. The risk of infection per act of vaginal intercourse was about 1.7 per thousand or roughly once in every 500 times. The risk rose dramatically with higher viral load in the infected partner. Younger people had a greater risk per act, and not surprisingly, had more frequent sex. Genital ulcers were strongly associated with transmitting, and lack of circumcision, genital ulcer disease, and herpes simplex type 2 seropositivity were also strongly associated with becoming infected.
The implications of the study are interesting. Possible methods of reducing transmission in populations where condom use is difficult to achieve include decreasing viral load, suppressing HSV-2 infection, and circumcision.
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