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Vaginal sex and HIV

Oct 21, 1996

Statistically, what is the probability of acquiring HIV from one act of vaginal sex with an HIV positive person if one is not using a condom... for the guy, that is?

Response from Mr. Sowadsky

Hi. Thank you for your question.

I have only heard of one report that tried to quantify the risk of HIV transmission through a single act of intercourse. The source is "K.M. Stone, HIV, STD's, And Other Barriers. In Barrier Contraceptives, Current Status and Future Prospects, 1994". According to this source, the estimated chance of infection from one partner to the other during a single act of intercourse is 0.1% to 20% for male-to-female transmission, and 0.01% to 10% chance for female-to-male transmission.

You will note several things on these statistics. Notice the wide range of variability in percentages (like 0.1% to 20% in the male-to-female group). Also note that male-to-female transmission is more likely than female-to-male transmission, but there is some overlap in the percentages. Although male-to-female transmission is more likely, transmission either way through vaginal intercourse can definetely occur.

Before you go out and say, "hey my risk is only up to 10%"....BEWARE! These statistics CANNOT be used to determine individual risk! There are many, many factors that determine your personal chances of becoming infected. For example, the longer the time that you have intercourse, and the rougher the intercourse, the greater your chances of infection would be. If the woman is having her menstrual period, the man's chance of infection would go up, since he would be exposed to both her vaginal secretions and her blood, both of which contain high concentrations of HIV. Also, the less lubrication that exists in the vagina, the greater the risk of infection would be. This is true since there would be a greater liklihood of microscopic abrasions on both partners for HIV to enter the bloodstream, which is required for infection to occur. In addition, if you were to have another STD where there are open lesions (like herpes, syphilis etc), your risk would go up substantially, since these lesions would make it easier for HIV to enter your bloodstream.

So, basically, in real life, these estimated infection rates CANNOT be used to determine your personal chances of infection. There's so much variability in the chances of becoming infected, that statistics like these cannot 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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