How does PrEP stop new HIV infections?

How does PrEP work, and is it really behind the reduction in HIV cases?
21 February 2017

Interview with 

Sheena McCormack from University College London


There's an HIV prevention measure called PrEP that looks like it could be responsible for a huge decline new infections, but how does it work? Sheena McCormack from the University College London conducted the PROUD study investigating the effects this drug could have, and she spoke to Kat Arney...

Sheena - PrEP stand for Preexposure prophylaxis, and prophylaxis is prevention, and preexposure is self-explanatory. So it’s a prevention you take in advance of being exposed to HIV and we are, as Chris mentioned, using antiretroviral drugs and anti-HIV drugs as PrEP. There is only one drug at the moment that’s licensed for use as PrEP and that’s a drug called Truvada and it does exactly what Laura explained, it stops HIV multiplying and that gives the body a chance to get rid of the infected cells when they’re caught in sexual fluids. So that’s how it works.

Kat - What are the side effects?

Sheena - We’ve used the drug for treatment for a long, long time so we are already quite familiar with the side effects and it actually very, very well tolerated. Only about 5 or 10 percent of people will ask to change that drug and they tend to be indigestion side effects very early on. The only side effects, I guess, we worry about in people who are taking the drug who have HIV are the longer term side effects in kidneys and potentially reducing bone density. But PrEP is something that we anticipate will take for a much shorter period of time. A sort of period of time like Greg explained when behaviours just change in that period before somebody can get things together again. It’s very unlikely that there are going to be side effects that really matter with shorter term exposure.

Kat - What did you find in your study when you were looking at the effects of this drug and what were you doing there?

Sheena - Our study was a bid different to the ones that had gone before. There’d been a study in gay men before that finished in 2010. They compared the PrEP drug Truvada to a dummy pill (placebo), and they showed a 44 percent reduction in HIV which is very exciting, but that is only partial protection. Something we were worried about in the broad community in the UK was if you gave people a pill that partially protected them, might it mean they then would abandon all other methods to reduce HIV infection, including condoms. If you threw away the condoms, maybe 44 percent would drop to 20 percent.

So we had to do a study a different way where we had a control group that knew they weren’t on PrEP, and a group who were on Prep who knew they were. So we randomised people to get PrEP straight away in the first year or to get it for the second year after a year of no PrEP, and that gave us the chance to compare PrEP to no PrEP. What we found, to our surprise, two things. First of all the rate of HIV was much, much higher than we expected in those not on PrEP. It was 9 percent per year and that is like 18 times higher than the general gay population, and PrEP reduced HIV by 86 percent. So much better than the dummy pill trial.

Kat - So, as you say, it’s not complete protection, so this isn’t a chemical condom, is it? People can’t just take it and go “I’m fine for everything now - let’s party”?

Sheena - Well Kat, it’s funny, in our study we only saw three infections in PreP users and in all of those individuals the story sounded like the weren’t taking the drug at the time of exposure.

Kat - Although, obviously, HIV is not the only sexually transmitted infection that you can pick up.

Sheena - Exactly. It only protects you against HIV and it’s not absolutely perfect. I think it would be unreasonable to expect anything biological to give you absolutely perfect protection. But the number of breakthrough infections that we’ve seen with PrEP have only been a handful so far so it is extremely good biologically. But it still is going to depend on people taking it in a period of risk.

Kat - And as we heard from Greg he went to get tested so he could get it and then discovered he had HIV. Quite briefly, is there a risk that people won’t be able to get it because they are carrying the infection and then also giving it to people with the infection might get HIV strains that are resistant?

Sheena - The biggest risk of that if the virus is multiplying like mad. If you’ve got that acute infection that Laura was describing and you give a little bit of drug, then you do increase the chance of resistance developing. So we want to test people, obviously, and make sure we know their status before starting PrEP. But the chance of somebody coming on the day they are acutely infected is, actually, pretty low. And, I have to say, in the study we didn’t do that we gave PreP straight away and we didn’t really see problems with following that particular path.


Add a comment