Numbers - hard to believe
Aug 7, 2005
You mentioned, in a recent post, that the estimated per-act risk for aquiring HIV from receptive vaginal intercourse with a partner confirmed to be HIV+ is 6.5 per 10,000 episodes. In another post, you mentioned that the odds of acquiring HIV from receptive anal intercourse with someone of known HIV+ status is .13% or 13 per 10,000 episodes. I am assuming (based on your commments in these posts) this data envisions the scenario where the penetrating partner ejaculates inside the recipient. That seems to say the AVERAGE person would need to have unprotected receptive vaginal intercourse 1538 times (769 times for receptive anal), with someone of know HIV+ status, in order to contract HIV.
Is it really that hard to get infected?
I ask because I recently had brief (partial penetration, no ejaculation) receptive anal incourse with a partner of unknown status. Presumably, this is considerably less risky (unknown status, no ejaculation) than the .13% described above. I have no intention of ever doing this again (too much anxiety, risk of other infections, etc). . . but was the risk of HIV infection really that low?
Response from Dr. Frascino
It is true that HIV is, thankfully, more difficult to acquire than some people may think. However, it's important to point out that estimated statistical risks should not be interpreted in the manner described in your question. If a particular sexual activity has a risk of 1%, that does not mean that some can participate in that activity 99 times before having to worry. Rather, statistical risk applies to each episode of that activity. In other words, every time someone does this particular activity, there is a 1% change he or she will acquire HIV. Yes, it could happen on the very first episode! The next time he or she participates in the same activity, there will also be a 1% chance of contracting the virus.
Receptive anal sex with an HIV-positive partner has the highest estimated statistical risk of acquiring the virus. Your risk "partial penetration," no ejaculation, unknown status of partner would decrease the statistical risk, but not completely eliminate it. Testing at the three-month mark is recommended.
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