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# Where The '1 in 10,000' Comes From

Feb 25, 2011 - 1 comments

Many of the posters in the HIV Prevention Community get hung up on the 1 in 10,000 figure often cited for the risk of receptive oral sex (and the 1 in 20,000 figure for the risk of insertive oral sex). But where did these numbers come from? It seems that a lot of people seem to think that there was some kind of massive study involving thousands of people where transmission of HIV was observed in 1 in 10,000 acts of oral sex. Not even close.

The numbers come from the journal article, "Reducing the Risk of Sexual HIV Transmission" by Varghese et al. (link to the paper below):

Here is a direct quote from the article that explains how the number were calculated:

"Although there are few data on the relative risks associated with fellatio (oral-penile contact), most investigators suggested that it is safer than vaginal sex. We assumed that insertive fellatio was 10 times less risky than insertive vaginal sex and that receptive felatio was 10 times less risky than receptive vaginal sex, per act."

In other words, since the risk of receptive vaginal sex is estimated at 1 in 1,000, the risk of receptive fellatio was assumed to be 10 times less risky- 1 in 10,000, and since the risk of insertive vaginal sex is estimated at 1 in 2,000, the risk of insertive fellatio was assumed to be 10 times less risky- 1 in 20,000. In Table 1 of the paper, the risks of fellatio have an asterisk next to them, with the asterisk qualifying them as "Best-guess estimate".

In science, risk assessors will use 'fudge factors' when they are required to extrapolate from a figure that is based on data to one for which no data exist. For example, in toxicity testing, toxicologists will arrive at 'safe' levels of a substance in part by extrapolating from the lowest dose that is observed to produce an adverse effect in the test animals, and apply "uncertainty factors" (which are usually also 10x) based on the characteristics of the study. The use of 10 as an uncertainty factor is convenient for scientists because it represents an order of magnitude. The risk assessors in the Varghese paper are doing something similar, except that they are just assuming that oral sex is 10x less risky than vaginal sex.

The paper came out in 2001, and since then there have been several well controlled studies that have failed to show any instances of transmission of HIV through oral sex. If you're still confused about these numbers, I encourage you to read the paper.

J.