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HIV and the Global Sex Tally

The views expressed are those of the author and are not necessarily those of Scientific American.


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Every year 2.5 million people are newly infected with HIV. This figure is simultaneously catastrophic and hard to understand, but the silver lining in that viral cloud is that we can use it to calculate how much sex people are having.

Here is how.

First we need to estimate how many people were infected with HIV through sex. To do this, we will leave out of our tally the 10% (or 0.25 million) people infected via injection drug use and the 0.3 million babies infected yearly during gestation, delivery and breastfeeding. That leaves around 1.9 million people who turned HIV-positive through sex every year.

Given that the risk of HIV transmission per sex act averages 1 in 900, for those 1.9 million instances of HIV infection to occur then people with HIV had to engage in – I’m guessing they were delighted to engage in – around 1.7 billion sex acts per year.

A caveat: not all sex is created equal when it comes to HIV transmission. The CDC estimates that the risk of HIV transmission is 1 in 200 for receptive anal intercourse, 1 in 1,500 for insertive anal intercourse, and 1 in 2,000 for penile/vaginal intercourse. That means some sex acts are riskier, or safer, than others. Both receptive and insertive oral sex are associated with HIV transmission – for instance one of my patients was infected with HIV after a year of offering fellatio to anyone who wanted it at a local bathhouse – but the risk is low enough that we cannot calculate it precisely.

For some reason we have not been able to find a thousand people willing to let epidemiologists sit at the foot of the bed to count up what proportion of sex acts are oral, vaginal, anal or other, so we really can’t make a completely precise estimate of the total sex acts among people with HIV each year. But it’s a large number, and it’s somewhere in the neighborhood of 1.7 billion.

Next, we should convert all that sex among people with HIV into the number of sex acts undertaken by the world’s total adult population in a furious and dedicated celebration of the biblical biological imperative to be fruitful and multiply and to have fun while doing it.

Assuming for a moment that people with HIV have sex as frequently as people without HIV, all we need to know to make that conversion is the number of adults in the world with and without HIV. UNAIDS estimates there are 31 million adults with HIV. Since 74% of the world’s population is over the age of 15, and there are around 7.1 billion people in the world, that means there are around 5.3 billion adults walking around thinking about sex with some regularity. Since 31 million in 5.3 billion, or about 1 in 171 of the world’s adults are infected with HIV, we can multiply the number of sex acts occurring among adults with HIV by 171, and, poof of smoke and wave of the hand, we have the Global Sex Tally.

Drumroll, please.

Globally, come rain come shine, come famine or flood, there are 291 billion sex acts every year.

Phew.

Moment of silence.

That is a lot of makin’ whoopee.

But wait, there’s more.

These calculations are predicated on the assumption that we can convert the number of sex acts occurring globally every year among people with HIV into the number of sex acts occurring in the whole adult population. But it is possible that people with HIV have sex more frequently than those without HIV, which would make our Global Sex Tally too high. Since I have patients who contracted HIV after their first and only sex act and others who have no idea which of hundreds of contacts infected them, I am loathe to make a facile conclusion.

We would need our sex-pidemiologists to conduct their nocturnal investigations among a thousand people with HIV and a thousand people without HIV and make some formal statistical comparisons in order to solve that question. So far the cash-strapped National Institutes of Health hasn’t prioritized that work. Even with those valuable figures in hand we might still wonder if they would be different in Topeka vs. Rangoon.

Perhaps the prevalence of certain HIV transmission behaviors is different among the HIV-infected population. This too could influence our estimates. Maybe we should compensate for the fact that anal sex is more common among men who have sex with men (MSM) than heterosexuals and thus a likely overrepresented sex act among the HIV-infected population. Since anal sex is more likely to transmit HIV, then this means our Global Sex Tally could actually be an underestimate. I for one am rooting for this possibility.

How should we factor condoms into our calculations? Condom usage is higher (but imperfect) among people who know they have HIV infection, and condoms reduce the likelihood of HIV transmission, so we could be concerned that our Global Sex Tally is dramatically short of the mark. This would be catastrophic. However, the 1 in 900 risk of HIV transmission per sex act is derived from a study in which subjects were taught to use condoms so I think it is a real world estimate and I’m sticking to it.

Regardless, there are many reasons to incorporate a margin for error in our Global Sex Tally. So, for the more precise-minded reader who wants to excite cocktail party interest in a statistically accurate manner, I suggest saying that every year the human race engages in 200-300 billion sex acts. Of course the natural question to ask in reply is how many sex acts that means the human race has committed in the history of mankind? I have no idea – that’s going too far.

Images: Anssi Koskinen and Eva Rinaldi

Tim Lahey About the Author: Tim Lahey, MD MMSc, is an HIV doctor, an associate professor at the Geisel School of Medicine at Dartmouth, and chair of the bioethics committee at the Dartmouth-Hitchcock Medical Center. Follow on Twitter @TimLaheyMD.

The views expressed are those of the author and are not necessarily those of Scientific American.






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  1. 1. CherryBombSim 10:07 pm 10/13/2013

    No. You cannot conclude that the 1.9 million people who contracted HIV engaged in 1.7 billion sex acts per year. Besides the fact that this is an absurd total of 900 sex acts in one year per person, it is simply fictitious math. All you can really deduce is that there must have been a minimum of 1.7 billion sex acts between an infected person and a non-infected person in the entire world that year.

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  2. 2. tlahey 6:03 am 11/23/2013

    That’s a great phrase: fictitious math. Love it.

    The facts, not fiction, are:

    - we know the number of people newly infected,
    - we know the average per sex act risk of infection

    Therefore we can, presto magic, estimate the total number of sex acts which led to all that HIV transmission.

    Simple math.

    Now, not everybody will experience that number of sex acts in an average year. Or even close in some cases. Which means there are of course some confidence intervals around those estimates. As discussed, see above.

    But the take home point is incontrovertible: there’s a lot of sex going on. And I estimated how much. This is a scientific service of historical proportions. In the years ahead I expect awards and honors. You can thank me later. Actually, the thanks I want is to have the variable describing all that sex named after me. Avogadro who? Introducing: the Lahey Number.

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