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It's hard to believe your statistics!! (HIV STATISTICS)

Apr 23, 2007

Hey Dr. Bob, I first read your site about 3 years ago when I was a sophmore in college and worried about a high risk HIV exposure (turned out negative). Today, I am a first year medical student and hope to eventually make half the impact you do on peoples' lives. I do have a question for you though. Often times you'll quote a risk being 1-4 in 10,000 or .5 in 10000 or something like that. To some that may sound equivilant to the probability of getting hit by a bolt of lightening while holding hands with a left handed lithuanian. How can it be true that engaging in unprotected vaginal intercourse with someone known to be HIV positive carries such a low risk that statistically you must have sex 10,000 times before you may EXPECT to become infected. Most happily married couples who have crazy mad grab-the-sheets sex don't do it 10,000 times over their entire lives!

Response from Dr. Frascino

Hi Med. Stud.,

Welcome back to the forum. Chances are as a first-year medical student you'll be learning a great deal about statistics and how to appropriately interpret them in a medical context. In general I would prefer not to give a specific number when explaining HIV risk, as this can indeed be very misleading to those who don't understand the limitations of statistical-risk estimates and attempt to apply these statistics in absolute terms to their individual situations. It's important to point out HIV transmission involves many variables other than just "route of exposure" statistical risk estimates. Factors, such as viral load, viral strain, host immune integrity and concurrent infections, as well as a variety of other factors, come into play for any specific situation. That is why I try to impress on folks these statistics are mere estimates. They provide information about general risk levels for various sexual activities, but should not be considered the absolute total and exact risks applicable to their specific situations. There are far too many variables (viral factors, host factors, situational factors, etc.) to accurately compute those numbers.

Statistics are not always easy to comprehend. For instance, when you flip a coin, the chance it comes up heads is one in two, right? Some folks could infer from that statistical chance that if they flip the coin twice it will come up heads at least once. This is obviously not true. In fact, even if you flipped the coin and it came up tails 10 times in a row, the chance your next flip will come up heads remains exactly the same as the first flip, one in two. The chance of coming up heads doesn't increase because you just flipped a bunch of tails. Similarly, with the HIV statistical-risk estimates of, say, 1 in 10,000, that doesn't mean you can perform that particular sex act 9,999 times before you have to worry about contracting the virus.

I should also point out the statistics I quote are really not "my" statistics, but those published by the CDC (Centers for Disease Control). They are estimated per-act statistical risks for acquiring HIV, based on the epidemiological studies published in the medical literature. Their purpose is to give some idea of relative risk of various sexual exposures. So if unprotected receptive oral sex with a poz partner carries a 1-in-10,000-exposures estimated statistical risk and unprotected receptive vaginal sex with a poz partner carries a 10-per-10,000 exposure risk, the important fact is that it's about 10 times more risky to have unprotected vaginal sex compared to oral sex. Similarly, the statistics would indicate it's 50 times more risky to have unprotected receptive anal sex and 9,000 times more risky to get a blood transfusion with HIV-tainted blood. However, the WWWs (Worldwide Worried Wells) are often not at all satisfied with "relative risk" figures. They want their specific risk, which as I mentioned above is nearly impossible to accurately calculate due to the number of variables involved. Also is a group of folks who are so irrationally worried about becoming infected they almost seem to fear they can catch HIV in the mail. In general HIV is much more difficult to acquire than most people think. Most folks are shocked to realize that even an exposure like mine, where I was stuck by an HIV-contaminated needle, carries a risk of only 1 in 300! Again this may seem like a low number just like the seemingly low numbers for various types of sexual risks.

And so we are left with three take-home messages:

1. HIV can occur even with a single exposure, even if the estimated statistical risk seems low. There are 45,000,000 of us who can attest to that fact.

2. Different sexual activities carry different levels of relative risk for contracting HIV. Unprotected receptive anal sex carries the highest risk.

3. HIV is probably not as easy to acquire as many people fear. Over twenty-five years of epidemiologic statistical research have provided us with consistent data related to these relative risks.

These messages can sometimes be difficult for folks to understand, because they are not intuitively obvious and may even seem contradictory. However, in reality, they are not. It's all in the interpretation and understanding of facts in the context of "crazy mad grasp-the-sheets sex."

Dr. Bob

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HIv Negative

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