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AIDS Transmission Rates

Nov 18, 1996

Can you tell me what would be the risk for the transmission of AIDS in a single, unprotected, vaginal sexual encounter between: an infected man and a healthy non-infected woman an infected woman and a healthy non infected man Please let me know what the possibility of infection would be in numbers (100%, 50%,etc.), I already know it's very high. thanks, Marga

Response from Mr. Sowadsky

Marga, thank you for your question.

I have only seen one report where a researcher estimated the risk of infection in a single act of unprotected intercourse. Their estimates were that in a single act of vaginal intercourse, the chances of HIV passing from male-to-female is 0.1% to 20%, and from female-to-male, 0.01% to 10% (Source: K.M. Stone, HIV, STD's, and other Barriers. In Barrier Contraceptives, Current Status and Future Prospects, 1994). THIS STATISTIC CANNOT BE APPLIED TO DETERMINE INDIVIDUAL RISK! This is because there are many, many variables that can determine whether a person becomes infected or not! Nobody can statistically determine in real-life circumstances what the chances are of someone becoming infected in a given situation. For example, the longer that intercourse takes place, and the rougher the intercourse, the greater the chance of transmission. Also, if the man ejaculates, the risk of infection increases significantly, since the woman would be exposed to both pre-cum and semen. The more infectious body fluids a person is exposed to, the greater the risk of infection there would be. In addition, if one partner were to have an STD where there are open lesions (like herpes or syphilis), the risk of infection would go up, since it would be easier for the HIV virus to enter the persons bloodstream. What the statistic does generally tell us is that it generally is easier to transmit HIV from male-to-female, than from female-to-male. But notice that there is overlap in the ranges of risk estimated. These statistics can give us a GENERAL idea of level of risk, but individual risk can vary dramatically, and statistics such as these CANNOT be used to determine individual risk.

If you have any further questions, please feel free to call the Centers for Disease Control at 1.800.232.4636 (Nationwide). Rick Sowadsky MSPH CDS

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