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Read more: 鈥�Instant Expert: HIV鈥�

For years the public health establishment has pontificated about 鈥渆vidence-based鈥� approaches to HIV prevention; we should only do what has been proven to work. The trouble is that things that succeed in careful scientific studies can fail miserably when translated into real life.

Medical science gives us evidence about what works technically. But we must also consider evidence about how prepared people are to change their behaviour. We need to factor in politics 鈥� what we can sell to governments and communities. And we mustn鈥檛 forget economics: what鈥檚 affordable, and what is not.

When considering a new intervention, we often cherry-pick the evidence that best confirms our own convictions. In assessing abstinence, religious leaders point to the medical science: no sex equals no sexually transmitted infections. Public health doctors, on the other hand, point to the behavioural science: promoting abstinence does not equal no sex.

When it comes to condoms, doctors cling on to medical studies showing they stop transmission of HIV and ignore the behavioural studies that show that condoms often fail because, except in commercial sex, people prefer not to use them.

No amount of evidence will make a programme work if, because of the politics, you can鈥檛 get it up and running. Needle exchange programmes, for example, are very effective at cutting HIV among drug users but they are often unpopular with voters and therefore politicians.

鈥淣eedle exchange schemes are a highly cost-effective way of slashing HIV rates among drug users鈥�

Another case in point is early antiviral drug treatment for people who are still healthy. While not necessary for their own health, the drugs cut the risk of them passing on the virus to their sexual partners. The evidence suggests this strategy works, at least in a tightly controlled research setting. But in a world already straining to afford drugs for people who have clinical AIDS, how can we afford treatment for the much bigger numbers who are HIV-positive but not yet ill?

Obviously, any prevention approach must work biologically. But we need to spend more time understanding how behaviour, politics and economics undermine or contribute to success. HIV prevention can only succeed on any major scale if it works on all four levels.