After 20 years of failure, HIV vaccine researchers have reported their first success in a clinical trial. What is different this time?
We don’t really know. The two vaccines given in combination to 16,400 volunteers in Thailand had each failed on its own. But somehow they worked well enough together to cut the HIV infection rate by 31 per cent. Many Aids experts had been doubtful about the strategy and some even questioned the ethics of the trial. Now they’ll be working intensively to understand the results.

But can we trust the positive results?
The bodies collaborating on the RV144 trial in Thailand – including the US Army and National Institutes of Health, the Thai health ministry and Sanofi Aventis, the giant French drug company – could hardly be more reputable. They insist that the trial was designed and conducted to rigorous standards.