After decades of failure, HIV vaccine developers have finally chalked up a success. A vaccine tested in northern Thailand appears to reduce the risk of infection with HIV by about a third. For public health officials, it is the worst possible type of good news.

Just a year ago, after the failure of yet another jab, some of the biggest names in HIV prevention were saying we should stop throwing money down the black hole of vaccine research. It is ironic, then, that the successful Thai vaccine is a combination of two previous failures. Some of those same big names had objected to the trial going ahead at all. They argued that it was unethical to inject 8,000 people with something that was unlikely to protect them from HIV, and another 8,000 with a placebo that certainly would not protect them.

The study went ahead anyway, funded by the American taxpayer to the tune of $105 million and conducted by the US military and the Thai Government. People who got the vaccine combination were 31.2 per cent less likely to contract HIV over the three years after the jabs than those who got the placebo.

Yes, it is exciting that something has finally been shown to work, even a bit. And yes, it will encourage a new look at combination vaccines. So why do I say it is the worst possible type of good news? Because a vaccine that reduces the risk of infection by a third presents an agonising public health dilemma.

With most infectious diseases, reducing everyone’s risk by a third would make quite a difference across a whole population. But the problem with HIV is that it is both an infectious disease and a behavioural one. I can get it by sharing needles with other drug injectors, I can avoid it by using condoms every time I have sex. If I know I have been vaccinated, will that make me more likely to share needles, or less likely to use condoms? And if it does, will that change outweigh the 30 per cent reduction in risk that comes with the vaccine?

The question came up in 2005, when studies showed that circumcising men reduced their risk of getting HIV through heterosexual sex by 60 per cent. A stunning result, especially for the hyperepidemics of Southern Africa, but one that health authorities didn’t act on for years because they worried that circumcised men would enjoy what the wonks call “behavioural disinhibition” — that “I’ve been snipped, so I can toss the condoms” thinking.

Large circumcision programmes are just beginning, so we do not know what effect they will have on behaviour. We do know that after HIV treatment became universally available in this country, condom use in the gay community fell and new HIV infections rose, even though treatment greatly reduces the risk of passing the virus on. Could the same thing happen if people feel protected by a vaccine? Probably.

I think we should develop a vaccine even at “modest” effectiveness; for people who do not have any safe behaviour to undo, a 30 per cent reduction in risk is better than none. But I’ll be curious to see if the public heath nannies agree.

Elizabeth Pisani is an epidemiologist specialising in HIV prevention and author of The Wisdom of Whores

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