Receptive anal intercourse ("bottom")
According to the Guidelines for the Management and Post Exposure Prophylaxis of Individuals who Sustain Nonoccupational Exposure to HIV, ANCAHRD/CTARC Bulletin, February 2001, the risk of transmission as a result of receptive anal sex is somewhere between 1 in 125 to 1 in 31 or approximately three per cent. Other experts place the risk as low as 1 in 1300.
Infection can and does occur with just one unprotected sexual encounter. It's important to note that even though the risk is 1 in 1300, that one chance in 1300 can be the first time.
Insertive partner ("top") during anal intercourse
While the risk is about 10 times less than being the receptive partner, the risk is there and is real. And of course the same holds true...it only takes one time.
Vaginal intercourse - infected men to uninfected women
There are so many variables that impact risk it is very difficult to place an exact number or give you exact odds. Risk estimates range from 1:1000 to 1:100,000, again depending on variables such as viral load, number of sexual contacts, etc. The important thing to remember is that it only takes once regardless of the odds so condoms are a must.
Vaginal intercourse - infected women to uninfected men
Studies seem to confirm that the risk is quite low, much lower than male to female transmission. But again, a number of variables make it difficult to know the exact odds. Again, it only takes once so condoms are a must.
Admittedly the risk of transmitting HIV during oral sex is extremely low, but the risk is there and it is not zero.
Keep in mind that beside HIV, oral sex can transmit other sexually transmitted diseases such has herpes and syphilis.
Which type of exposures carry the highest risk?
Certain exposure routes do offer higher risk of infection than others. Those exposures that carry the highest risk include:
- Sharing needles to inject drugs
- Receptive unprotected anal intercourse
- Vaginal intercourse
Breatfeeding & Child Birth
There are two transmission routes that have very high incidence of HIV infection - upwards of 1 in 4.
- child birth - transmission from infected mom to her unborn baby
- breastfeeding - transmission from mother to baby during breast feeding.
Fortunately, the risk for these two transmission routes can be decreased dramatically by good prenatal HIV care, expert HIV care during delivery, and HIV care for the baby after the delivery.
Numbers and percentages really are just guidelines. Infection can and does occur after one unprotected sexual encounter or after sharing a needle once. The only way to know if you've been infected is to get tested. Percentages and numbers should not be a substitute for testing and safer sex.
- AIDS Infonet Fact Sheet #152 - "How Risky is It?"; 25 Oct 2005.
- Murphy, D. "Risk reduction strategies for anal sex with casual partners."; AFAO/NAPWA Education Discussion Paper. Vol. 2, No. 4, 2000/2001; 1 Jun 2001.