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cdc figures not scientificially based acc. hopkins institute

Aug 21, 2006

Hello dear DR. Bob! As a regular visitor to your website I noticed you follow the CDC guidelines or take them as reference; a specific question asked at the johns hopkins patientforum about the reliability of those often quoted figures was answered by Joel Gallant M.D. in a rather "demasking" way, and i may quote from this article: "I went to the CDC document, where their table states that the risk of insertive oral sex is 0.5 per 10,000 exposures, or 1 per 20,000 exposures. I then went to the original source they cited, the Varghese paper from Sexually Transmitted Diseases, to see where this number came from, since it was news to me. The authors state "Although there are few data on relative risks of fellatio (oral-penile contact), most investigators suggest that it is safer than vaginal sex. We assumed that insertive fellatio was 10 times less risky than insertive vaginal sex and receptive fellatio was 10 times less risky than receptive vaginal sex, per act." Thus, they categorized insertive fellatio as the safest of the sexual acts they were addressing, giving it a relative risk of 1. That makes it the comparator against which other acts were compared, so that receptive anal intercourse had a relative risk of 100, making it 100-times riskier. Then, in order to convert the relative risks to absolute risks, they "estimated" that the per-act transmission from an infected male to an uninfected female during unprotected vaginal sex was 0.1%, so the risk for all other sexual acts was then calculated base on the relative risk they had previously assigned, which is how they came up with 0.5 per 10,000. By the way, on their relative risk table, where they assign a relative risk of 1 to insertive oral sex, there is a footnote that says "best-guess estimate."

Whew! We're not talking precision science here, but rather estimates based on assumptions based on guesses. I'm not saying that there's no value to such an exercise--it's useful to have a sense of which acts are more dangerous than others, and by how much--but I wouldn't rely to heavily on the numbers that come from it.

But even if, by some stroke of pure crazy dumb luck, the authors were right and the risk of insertive oral sex WAS in fact 1 in 20,000, I wouldn't be too concerned. That's close enough to zero for me. However, I know that a good proportion of my Worried Well readers will view 1 in 20,000 as cause for hysterical, mindless panic, and they'll be typing away furiously on their keyboards only seconds after reading this post, asking me about blood and saliva and lack of circumcision and subtypes and tooth abrasions and God knows what else. And to them I say, type away, my friends: it's good exercise for your fingers. But don't sit by the computer waiting for me to respond". 1) So would you agree that the often cited stats of the CDC are not at all based on facts but mere assumptions and we should be more carefull in referring to them as the golden standard? 2)I can't help noticing that you after having a relationship for more than a decade; still talk about mind-boggling, toe-curling and wake-up-the neighbours sex; whereas for most people after such a long time it is more the "damn its again sunday and sex is sheduled"routine? Do you have a secret-recipe or is it just your "hot"itialian background? Dr. Bob you are great and please continue to mix the facts and figures with your very appreciated humor:)

Response from Dr. Frascino


1. Please note when I quote these statistics, I always note that they are an "ESTIMATED per-episode risk." Estimates by definition are based on assumptions. Personally I would prefer just to advise folks that their risk is either high or low. But everyone wants to get a handle on how high or how low. The CDC is one of the most conservative scientific agencies addressing HIV risk. Therefore I used their "best guess estimates" to try to address the monumental worries that devastate so many of the folks writing to me in desperation. At least for the time being I believe this information helps folks place their risk in a perspective that is better and more realistic than their perception of that risk, which is often blown way out of proportion.

2. Secret recipe? Nah. All you do is mix one hot Norski with one hot Italian and toes will curl, neighbors will awake and the Religious Right will get even more pissed off.

Dr. Bob

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