PrEP's total effect depends on the balance of behavioural change

Truvada is a pill which provides pre-exposure prophylaxis. That is, one gets to go do the risky stuff while reducing the likelihood of getting a nasty from doing the risky stuff. Put like that it seems pretty obvious that the taking of Truvada is going to increase the incidence of the risky stuff. Whether it increases or reduces the incidence of the nasties depends upon the extent of the increase in exposure to risk and the effectiveness of the prophylaxis itself.

One could imagine, for example, a 10% reduction in risk leading to a 20% increase in activity and thus a rise in nasties. Or, equally, a 99% reduction in risk, a 50% rise in activity and a fall in incidence.

The PrEP effect upon HIV incidence is “It depends.”

All of which rather means that we’re in economics here:

How PrEP, the pill to prevent HIV, may be fueling a rise in other STDs

Yup, we sure are.

Now there’s data showing that some concern is justified. A systematic review published in March in the journal Clinical Infectious Diseases found that some PrEP users are having more risky sex — and as a result, getting more sexually transmitted infections (STIs). The review, which brought together 17 studies on PrEP use and sexual behavior change, suggests that as people begin to trust PrEP, they’re having more condomless sex and worrying less about other STIs. The more recent studies in the review show the strongest trend.

It’s a timely finding for two reasons: PrEP is about to become more widely available, and cases of syphilis, gonorrhea, and chlamydia have lately been rising in the United States. Right now, PrEP is only available to people at the highest risk of getting HIV, including gay and bisexual men, people who engage in sex work, and people who use injection drugs. But the Food and Drug Administration just approved it for teenagers, and some experts think anybody who wants it should have access.

We’re very much in economics here.

So, we can replace all that about activity and nasties with elasticity. By how much does the amount of activity change in relation to the change in risk? We’d probably, because we’re talking about humans and sex here, insist that we mean perceived risk too. We’d also absolutely insist that elasticity changes over time – near every response we know of to anything becomes more elastic over time.

And we’ve that one more point. The purpose of Truvada is to allow sexual activity which wouldn’t be undertaken in its absence. Thus we really do rather expect what would be risky sexual behaviour in the absence of PrEP to increase in the presence of it. That’s why we’ve gone and made it in the first place.

We’ve more we can add to this as well. We know very well that the introduction of safety belts led to more risky driving. As was pointed out back at the time they were being made mandatory, we’d get less risky driving if instead of a 3 pointer we had a sharp knife mounted on the steering wheel – aimed at the driver’s heart. It’s simply true that if we reduce risk then humans feel safer and so take more risk.

Whether this means a reduction in harm overall, well, that depends. Seat belts, almost certainly yes. Bicycle helmets, quite possibly not. We do know that those who wear helmets while pedalling take on more risk. We don’t really know, as yet, whether that leads to a higher cyclist injury or death rate overall or not. We’re still in the world of duelling claims on that one.

Concerning PrEP we really are pretty sure that the prophylactic part of Truvada is effective enough that the incidence of HIV will fall near whatever sexual habit changes occur. Just as we’re equally sure that the incidence of risky behaviour will go up because that’s what Truvada is for. As to whether it’s all worth it, well, that’s not actually up to us is it? Consenting adults and all that, it’s entirely the decision of those who decide to indulge in the behaviour, take the drug, or not. More sex, more clap, less HIV.

Shrug, not our decision to take for them, is it?

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Rhoda Klapp
Rhoda Klapp

Would like to see another column of potential beneficiaries per 100,000 of population, because this implies that risky behaviour is different according to race, and that can’t be true, can it?


“PrEP is about to become more widely available” not because gay men will rush out and buy it in order to jump into bed with a partner with AIDS, but because the government is going to provide it. New Hampshire yesterday got federal approval to start providing clean needles to exchange with intravenous addicts; social workers referred to this as “meeting the client where he is,” letting him misbehave safely. (Except for all those other diseases.) There is no way to give benefits to individuals in a certain state without increasing the value of being in that state. Another question… Read more »


You see a similar argument in sports, the improvements in safety equipment means players will take more risks, e.g in ice hockey there are hits a player would have backed off from making before that they now don’t worry about.
I’ve actually seen pundits argue that removing safety equipment would make things safer