Risk of HIV

May 5, 2005 9:56 AM

What is the current scientific consensus on the risk of a man contracting HIV from a positive woman through standard sex?

The reason I am asking is because figures I find vary wildly between 1/3 and 1/200,000

A lot of websites seem to have an agenda, 'use a condom everytime or you'll die horribly' or 'aids doesn't exist it's just a pc lie..'

I just want a reliable figure so I can decide for myself, or at least to know what the current debate is.

No answers saying 'just use a condom' please I know there are plenty of reasons to do this with strangers and would always respect a partners needs. There are times between casual and long term relationship where I feel the need for a better idea of risk.

posted by anonymous to health & fitness (12 comments total)

The Straight Dope covered this in 1988. I don’t know if the figures he cites are out of date.
posted by hilker at 10:05 AM on May 5, 2005

anonymous posted "There are times between casual and long term relationship where I feel the need for a better idea of risk. "

Well, if you're only taking the risk in an on-going if casual relationship, you do know that there are now HIV tests the results of which are available in twenty minutes, right?
posted by orthogonality at 10:19 AM on May 5, 2005

I tried to google this probability a few weeks ago. The best I found is that the researchers could not locate enough 100% straight men with HIV to make a determination.
posted by mischief at 10:19 AM on May 5, 2005

There are too many variables to provide any sort of meaningful data. First off, you don't mention whether either of you have other STDs. The HIV transmission rate increases significantly if one or both partners are coinfected. In addition, it appears that there's a correlation between viral load and transmission (see, e.g., European Study Group on Heterosexual Transmission of HIV: Comparison of female-to-male and male-to-female transmission of HIV in 563 stable couples. BMJ 1992, 304: 809-813). Finally, there are other nonsexual practices that may increase the risk of transmission (douching and lack of circumcision, for example).

See this article for more information.

On the other side, see this JAMA article for results of a specific study.

Now that I've attempted to answer the question, please allow me to say this: even -- especially -- in a committed relationship, where sexual contact is presumably more frequent and consistent than in shorter-term ones, your risk of contracting HIV through unprotected sex is greater, since the chance of being infected is independent of the number of previous infection-free encounters.

You're right to do a risk analysis, but please consider whether you can live with the worst-case scenario.
posted by aberrant at 10:44 AM on May 5, 2005

It's probably also in flux because straight African-American woman are the fastest growing infected group in the US right now. So the percentage of straight women with HIV is growing rapidly.

And as the Straight Dope article pointed out, your risk increases GREATLY the more times you have intercourse. A one-time encounter may not be all that dangerous, but an on-going no-condoms relationship with the same person suddenly becomes much more risky.
posted by occhiblu at 10:45 AM on May 5, 2005

I'm trying and failing to find good stats, but in general (I am not a doctor) the chances are not huge if there's not blood or infected discharge (pus) present in the female, and not an open sore on the penis. By "not huge" of course I don't mean non-existant, but relatively smaller than the other way around, by far.

If the female has no blood present (and I'm not just talking menstrual, but there can even be microabsarions they can't feel that would allow blood to be present), and they don't have an STD that's creating pus (gonorrhea, chlamydia) OR an open sore in the vagina AND the male penile skin is 100% in tact (which, again, isn't necessarily something you can see, there can be microtears) THEN the chances are not great of transmission F to M.

IF, however, there's blood present in the F, AND there's an opening for the blood to enter on the M, then, yeah, of course there's a bigger risk.

Females are at greater risk of infection by males because there's such a great "deposit" of HIV in the semen or infected pus directly into the body. Females just can't deposit that amount of blood or pus directly into the male body. (If I can find stats I'll post them for sure)
posted by tristeza at 10:57 AM on May 5, 2005

There was a discussion here about STDs in circumcised vs. non, and someone linked to a study showing that the probability of HIV from infected femal to male is < half of 1 percent.
posted by eas98 at 11:29 AM on May 5, 2005

Here's a fact sheet document from the Center for AIDS Prevention Studies at UCSF. It dates from 2001, so it's pretty old in the AIDS world. You could contact one of the authors and see if there's any updates.

This is not an issue that much interests researchers, as the risk is pretty low, and they'd rather be spending their time working to bring down transmission rates among higher risk groups.

BUT - if you are looking for these data to help you make a personal decision about sexual activity, DON'T DO IT!!! Research articles speak about incidence in a sample, or in a population and have no relevance to particular sexual acts.

If the woman you're having sex with doesn't have HIV, the risk of you contracting it is zero. But it's exceedingly difficult to know whether a person has HIV or not. It's not visible, seroconversion takes time, etc. If the woman does have HIV, then the risk is not zero - but what it is in particular depends on what kind of sex you have, whether you have any open sores or wounds (which could be microscopic and not necessarily visible), whether you use protection or not, how virulent her strain of HIV is, her viral load, etc. Obviously, condoms serve as a protection, but they have a failure rate.
posted by jasper411 at 11:44 AM on May 5, 2005

ortho, the rapid HIV test still doesn't work if the antibodies aren't present in your bloodstream. That takes six weeks to three months to show up. While there is now a rapid emergency exposure test, it's only about 80% effective.
posted by dirtynumbangelboy at 12:32 PM on May 5, 2005

There are times between casual and long term relationship where I feel the need for a better idea of risk.

Keep in mind that the chance of infection changes with the length of the relationship, which is one of the reasons it is difficult to get an exact number. So, if you're having sex with someone who is HIV+ only once, it'll remain at that <1% or whatever. However, if you have a long term sexual relationship with this person, eventually your chance of infection will reach 100%. And besides, statistics aren't really a good way of describing risk in an individual situation. They only apply over a large population, and should never be used as justification. One way to look at it is, for every 100 people, 99 of them have no chance of getting HIV from unprotected sex. But one of them has 100% chance. And there is no way of telling which one of those categories you are.
posted by orangskye at 4:15 PM on May 5, 2005

A recent article said that infection risk is very high right after the transmitter is infected — many times higher, for a few weeks — it then drops (to the ~0.1% per act level) and later rises a bit in the last few years of the person's life. This was based on statistics gathered in Africa. Let's see: Science News 23 April '05, citing the May 1 Journal of Infectious Diseases. Your local university library probably carries the latter.

[soapbox] Do be aware that AIDS isn't the only STD out there. It's not even the only incurable one. The risks are cumulative with each disease and each act, which adds up fast. And consider that the people you're having sex with probably didn't think they'd get infected, either. [end soapbox]
posted by hattifattener at 5:47 PM on May 5, 2005

I just want a reliable figure so I can decide for myself

Well, if it's any help, *she* would be more likely to get it from you than you from her: "The efficiency of infection is much higher in male to female than female to male transmission." But seriously, have you thought about what percent risk would make it ok for you to have unprotected vaginal sex with someone whose HIV status you aren't sure of? 4%? 15%? 35%? How would you decide?

I'm not being judgmental here; just recommending you think about what the numbers (which are currently impossible to quantify, but let's pretend) might mean for certain specific behaviors. Are you going to go down on her without a dental dam? Let her go down on you? What if she has bleeding gums? Think as specifically as possible; terms like "standard sex" aren't enough.

For instance, back in the late 80s I decided not to worry about the risk from precum when giving blowjobs, but stopped letting guys cum in my mouth, even though I understood there was a small-but-possible risk from precum, too. To me, that risk seemed less than, oh, 2%, and that was ok, even as I accepted that I just might get infected and hate my own fucking stupidity someday. Your mileage may vary.

Anyway, the point is to get yourself thinking as clearly as possible about the level of risk from each specific behavior - knowing that the statistics are going to be iffy at best - and then look yourself in the mirror and decide what level of risk you're (for now, anyway) comfortable with. Fuck judgments from other folks; the only person whose judgment matters here is you.

One more thing re: open sores on the penis -

The opening at the end of the urethra is generally considered a perfectly adequate mucous membrane for HIV entry. From the same doctor linked above: "The presence of genital ulcerations increases the risk of transmission, but is certainly not a requirement." In other words, if she's infected, HIV can enter your body through the tip of your penis alone. Except if you're wearing an unbroken latex condom, of course.
posted by mediareport at 10:25 PM on May 5, 2005