It's been a bad few months for HIV prevention. We've learned that our best candidates for vaccines and virus-killing microbicides don't work. Now we're clutching at another straw: maybe we can treat our way out of the HIV epidemic.
At an HIV research meeting this week, boffins from the World Health Organisation revived a mathematical model that shows that if we test everyone in Africa for HIV once a year and give everyone who tests positive expensive drugs right away and for the rest of their lives, we'll wipe out new HIV infections within seven years. That's because HIV is passed on most easily when there's lots of virus in the infected person's blood and body fluids. Antiretroviral medicines cut the "viral load" (the amount of virus in the body), so they make it more difficult to pass on HIV. Ergo, more treatment means fewer new infections.
Sadly, it's not that simple. For one thing, HIV is most infectious in the few months after a person is first infected. Even if everyone got tested annually, we'd miss most of these new infections. Second, people's viral load spikes upwards if they get another sexually transmitted infection (STI), or if they stop taking their medicine because the clinic runs out of stock, the meds make them feel sick, or they went on a three-day bender and forgot their pills. Interrupting treatment also allows the virus to develop resistance to drugs, and that leads to more spikes in viral load. Most importantly, antiretrovirals keep you alive and well enough to be out there meeting new sex partners. That's a good thing, obviously, but it also means that people who have HIV are going to have more chances to pass it on during those times when their viral load is spiky.
There's more. In countries like the UK where treatment has been available for over a decade, Aids has virtually disappeared. HIV, unfortunately, has not. A few years after antiretrovirals became widely available, new infections among gay men in the UK began to rise. We've seen the same thing in Australia, the United States and practically everywhere else we have data. One reason for that is that gay men use condoms less now than they did when HIV = Aids = a horrible death. Now, though, HIV = a pill every day. Boring, but not the end of the world, unless you're the taxpayer picking up the tab for it or the epidemiologist worrying that drug-resistant strains of HIV will reignite Aids.
On top of that, many people assume that if the person they're having sex with is infected, they'll be on meds and so not very infectious. Which may be true if they're not in that early peak of infectiousness, have taken all their pills diligently, and don't have another STI. Though since condom use is dropping across the board, other STI rates are soaring. In short, more people living with HIV, combined with more unprotected sex is outweighing the effects of lower viral load in places where the population is well informed, HIV testing is actively promoted, and treatment has been free and universally available. But in Africa it will be different.
Our computer model assumes every African will get tested for HIV every year, everyone who tests positive will start taking antiretrovirals immediately and 98 out of 100 will never miss a dose. On top of that, though gay men in rich countries use condoms far less now than they did before we had antiretrovirals, we assume that heterosexuals in Africa are going to use them more once the most visible and frightening face of Aids disappears.
On the strength of this model, which bears as much relation to reality as an MP's expense claim, we are going to hail expanded HIV treatment in Africa as the new answer to prevention. A triumph of optimism over common sense.
