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From: york@mbcrr.dfci.harvard.edu (Ian A. York)
Newsgroups: alt.folklore.urban
Subject: Re: Aids myth - Now true
Date: 14 Sep 1995 22:34:24 GMT

In article <4377gs$4mq@news1.panix.com>, Angus Johnston <angusj@panix.com> wrote: >
>Hogwash. Hogwash, hogwash, hogwash. These numbers are several orders of
>magnitude higher than anything that's appeared in the literature. Not

Angus has already noted some of the logical flaws in the new AIDS Mary story; I agree with him, and the others who say this is likely "F" and vectored by the priest. Here I'm just going to talk about the odds of the putative lady infecting 5 out of 80 young gentlemen. I shouldn't have to say this, but I will: I'm not an HIV specialist or an AIDS clinician, and so don't treat me as one.

Surprisingly enough, I think the 5/80 isn't completely implausible. The risk of contracting HIV (female-to-male) after a single boff is very hard to get accurate numbers on. The literature is all over the place. The concensus seems to be "very small", or "less than 1%", or "much less than 1%", or 1 in 500, but the people who say this don't give very good reasons for it.

So let's take a look at some of the primary data. Relevant studies mostly look at cumulative risk of contracting HIV, in couples where one partner (female in this case) is infected and the other is initially free. I'll come back to whether this is directly related to the question at issue or not. I'm not going to post all of the studies; there's one handy-dandy review in JAMA 1992 vol 268:p 1856 which has a little clip-n-save type of table of 16 serological studies; the risks in these surveys ranged from 1% to 73%, with an average of 25%. A larger Italian study [1] found 10% of the males becoming infected. (I don't know if these couples were warned about the situation beforehand, or if they used condoms; I doubt they did use condoms because that should have dropped the odds much below this.)

On the other hand, another study [2] (which has not been universally accepted, but anyway) specifically looked at female-to-male transmission rates, and found surprisingly high rates: 3.1% per boff, with a worst-case scenario of 7.5% infection-per-boff. Note that this is Asian, and there is evidence that in Asia and Africa the virus is more readily spread through heterosexual sex than in North America (debated).

Are these studies completely incompatible? A quick calculation shows that if the risk is 3% per boff, then a mere 9 boffs or so will bring us to the 25% cumulative infection level, and just 3 - 4 take us to the 10% infection level. Now Angus is the married man here, not me, and I don't know how many years it takes a married couple to put in nine acts, but anyway, superficially, this sounds like these figures don't make a lot of sense. If the risk is 1% per boff, and if I'm punching in these figures right, then we find around 10 boffs -> 10% cumulative risk, about 30 boffs to the 25% cumulative risk; if the risk is 1/500 (0.2%) then it's respectively around 50 and 150 boffs. Incidentally, if any married or unmarried couples feel like bragging this is your chance.

So on the face of it, the 1/500 is much more in tune with the cumulative risk studies than the 3% risk-per-boff. But that makes the assumption that risk of infection per each act is independent, which is not the case in a couple (but may be in the scenario at question, with 80 independent boffs). This is purely speculative now, but I can easily come up with mechanisms of viral spread and infection that imply that the outcome of the previous boff, infection-wise, influences the next one. (I should warn you that my colleagues have admiringly told me that I can also make plausible viral scenarios that involve pink tap-dancing elephants.) I won't go into this here, but if you're really fascinated and want a nice little lecture on molecular viral pathogenesis, drop me a note.

OK, so now let's make the following assumptions: The woman was infected with an Asian strain of HIV, and so was more infectious than if it was a North American/European strain. The woman was in a highly infectious period - evidence suggests that early in infection the carrier is more contagious. This makes her in the same sort of group as the Thai study. Let's also assume that the upper end of the Thai study, for their underreporting scenario, is the real risk: 7.5%.

Five out of 80 is 6.5%.

I still don't believe the story, but I don't think the spread rates alone can be used to eliminate it.

Ian

[1] Nicolosi A. Correa Leite ML. Musicco M. Arici C. Gavazzeni G. Lazzarin A.
The efficiency of male-to-female and female-to-male sexual transmission of the human immunodeficiency virus: a study of 730 stable couples. Epidemiology. 5(6):570-5, 1994

"Among the 730 couples, 24% of the female partners were HIV positive, in comparison with 10% of the male partners. ...we found that the efficiency of male-to-female transmission was 2.3 (95% confidence interval = 1.1-4.8) times greater than that of female-to-male transmission."

[2] Mastro TD. Satten GA. Nopkesorn T. Sangkharomya S. Longini IM Jr. Probability of female-to-male transmission of HIV-1 in Thailand. Lancet. 343(8891):204-7, 1994

" ... we estimated the probability of HIV-1 transmission per sexual contact to be 0.031 (95% confidence limits [CL] 0.025-0.040). Allowing for random error in the self-reported frequency of contacts, the estimate was 0.056 (95% CL 0.041-0.075). ... These estimates are substantially higher than analogous estimates made in North America."

Ian "Mr. Boffo" York
--
Ian York (york@mbcrr.harvard.edu)
Dana-Farber Cancer Institute, 44 Binney St., Boston MA 02115 Phone (617)-632-3921 Fax (617)-632-2627