In a fascinating new paper published in the wonderful, open-access Public Library of Science, Fraser Lewis and colleagues shed a little more light on how HIV spreads. They show that, at least among gay men in the UK, HIV propels itself in bursts, exploding quickly through a whole cluster of people and then tailing off.
They also show that a quarter of all new infections occured within the first six months after a person was infected, and close to half in the first year. These data are hugely important. There’s a big push in the United States and some other countries at the moment to focus HIV prevention on people who are already infected. This makes some sense; only people who have the virus can pass it on, so if they could be persuaded to use clean needles and condoms the problem would be solved, right? Yes but. The but here is that if you take this approach, you can only focus on known positives. Most people in most countries aren’t diagnosed until they’ve been infected for many years. And as this new study confirms, by that time they’ve already infected most of the people they are going to infect.
We’re back to something I’ve posted on before: the peaks and troughs of our sex lives, aka our slutty phases. Most infection gets passed on soon after you’re infected. And very often you get infected when you’re climbing one of your sex life’s peaks, when you are out there having a good time, often with several partners in just a few months. (Aside — how come the troughs always seem to last so much longer than the peaks?). Lewis isn’t actually looking at behaviour, he and his colleagues base their analysis on genetic similarities and differences in the virus itself, but the conclusion is inevitable.
In my mind, the clear implication is that we need to focus HIV prevention efforts in physical places and in communities where we are likely to intercept people in their slutty phases — in clubs and bars, on internet dating sites and in chat rooms. Waiting until someone is diagnosed as infected before cranking up prevention is just not good enough.
By the way, if you’re intimidated by the nerd-speak in the paper itself (or even by its title: Episodic Sexual Transmission of HIV Revealed by Molecular Phylodynamics), just read the Editors’ summary. I cannot commend PLoS too highly for these wonderful, accurate but extraordinarily user-friendly summaries. They represent a huge step forward in the public communication of science.