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PaperclipPerfector  2mo ago (text post)   2763 thread views

Small-Scale Question Sunday for September 8, 2024

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A medical question. I've often heard it said that one is far more likely to contract HIV as a result of gay sex than straight sex. Is this statement true purely as a result of the incidence of HIV infections in the gay male community (i.e. the proportion of gay men who have HIV is vastly higher than the proportion of straight people) - or is there also a mechanical component that makes the virus more likely to spread via certain sexual activities than others? That is to say, supposing Alice and Bob both have HIV, and they've both had it for the same duration, neither are taking PReP. Is Carl more likely to contract HIV as a result of having anal sex with Bob than as a result of having vaginal sex with Alice? Moreover, if Carl had anal sex with Alice, would he have the same risk of contracting HIV as he would if he had anal sex with Bob? Lastly, does it make a difference who the receptive partner is - is Carl more or less likely to contract HIV from Bob if he's the bottom than if he's the top, or does it make a difference?

Semi-related: in African countries in which it was and is predominantly straight men and women with HIV (to be clear, gay men in these countries still have it at higher rates, but they are not the majority), it was predominantly prostitutes who served as reservoirs for the virus. While the rate of infection was low, it was high enough that, over time, ever larger numbers of straight men (and then their wives and eventual children) became infected.

Lastly, does it make a difference who the receptive partner is - is Carl more or less likely to contract HIV from Bob if he's the bottom than if he's the top, or does it make a difference?

Yes, tops rarely contract HIV through performing anal sex on bottoms (though it is thought to be marginally more common in uncircumcised men than circumsised ones.) Bottoming is the main cause of contracting HIV among men who have sex with men.

I don't know if there is any research on whether a woman receiving anal sex is more, less or the same riskiness as a man receiving anal sex in terms of HIV transmission rates but I'm curious to know the answer.

Unfortunately, most studies specifically looking at heterosexual anal intercourse are either explicitly or implicitly about Vulnerable Populations, which leaves little reason to trust it can be meaningfully compared with the typical study of gay intercourse. Or they just use gay numbers directly for estimates.

There's mechanical and social reasons that it could be riskier for bottoming men (guys on average like it rougher, the prostate exists) or women (less likely to stretch themselves recreationally, selection effects on partners), but it's probably not a knowable number, and my gut check is that it's probably not an orders of magnitude difference.

The mechanical elements are a major part of it, but the social element is probably larger. I just finished And the Band Played On, which was excellent, and I have an effortpost in my head about the characters that pop back up like bad nickels.

But something that runs through the early years of AIDS is the process by which they kept trying to shut down, or at least soften the activity in, Bathhouses in SF and NYC and other cities, and they didn't do it. There is simply no hetero equivalent, none, for that kind of normalized, regular, random, high partner count sexual activity outside of a brothel. Not all gay men used the bathhouses, but basically all knew people who did.

While there are heterosexuals who engage in that kind of sexual athleticism, they tend to cluster together: whores and whoremongers, rock stars and groupies, etc. So for a random heterosexual, you're fairly comfortable that your partner has no connection to that kind of thing. For a random homosexual in the 1980s, unless you're talking to a virgin the odds are that he's no more than a few degrees of sexual separation from the bathhouses.

Anal is way riskier than vaginal.

What follows was learned over a decade ago in microbiology class and may be out of date.

HIV exclusively infects cells of the immune system through a handful of receptors, none of which are expressed on the mucosa of the anus/vaginal tissue. As a consequence, it needs to penetrate multiple layers of mucus and epithelial tissue before it can reach a cell that it can productively (use to produce more viral particles) infect. Anal sex generates microtears in the mucosa much more readily than vaginal sex and provides more opportunities for the virus to reach the bloodstream/immune tissues. There was also some speculation about 'sensor' immune cells that reach into the epithelium that may also act as a route for infection, but I'm skeptical.

What shocked me in that class was just how rare transmission was; you can see the numbers in the table morgenland linked. Made me think that you have to be either extremely unlucky, extremely promiscuous or just stupid/desperate enough to share needles to get infected.

There are differences depending on the type of intercourse, for sure. Look at the table 1 in this paper. It estimates receptive anal intercourse to be about 17 times riskier than receptive vaginal contact. Also, there is a demographic of homosexual men known to live rather promiscuous sexual lives.

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