The need for a vaccine can hardly be understated. There are 40 million people infected with HIV in the world - almost 30 million of them in impoverished sub-Saharan Africa - and the numbers are rising exponentially. The pandemic has not yet peaked and is not expected to do so until perhaps 2050. Huge efforts have gone into prevention and a lot has been achieved in countries such as Uganda and Thailand, but even there, it does not look as though the infection rate has permanently stopped rising. In many other countries, customs, traditions and cultural taboos have to be overturned before people can feel able to use condoms and practice safe sex with their partners.

So the need is pressing. All credit must go to VaxGen for pushing ahead with its vaccine trials, which are a lengthy and very expensive business. The phase-3 trial - the final test of whether a vaccine prevents HIV infection in those most at risk of it - ran for three years in the United States, Canada, Puerto Rico and the Netherlands. The interim results had not been encouraging, but VaxGen pressed on, led by its determined founder, Dr Donald Francis, an epidemiologist and virologist who once worked for the Centres for Disease Control and Prevention in the US and has been involved in the fight against Aids from its alarming beginnings in the 1980s.

Is the news from VaxGen completely disastrous? No. First, any mass trial produces data that will be useful for future attempts, and VaxGen has said it intends to carry on with the work on the vaccine. Second, there is a strange quirk to the results, in that, while there was no overall difference between the proportions of high-risk volunteers (over 5,000 men who have sex with men and 309 at-risk women) given the vaccine and those given placebo who became infected with HIV, there appeared to be a difference between ethnic groups. The vaccine seemed to have a protective effect among African-Americans (78% fewer infections) and other non-Hispanic minorities (67% fewer infections).

The big cautionary note to this is that the numbers of black and other ethnic minority volunteers were very small. Out of a total of 5009, there were 314 blacks and 498 from other minorities. While the scientists report that the findings in these groups were statistically significant, the analysis is based on just 13 infections among the black volunteers - four of whom had been given the vaccine and nine of whom had not.

IAVI (the International Aids Vaccine Initiative), set up to fund and promote trials, says that it is difficult to draw conclusions from these figures, which need to be further analysed. VaxGen took comfort from them, though, and announced that it intended to head towards a licence, after conducting any necessary additional studies, for the vaccine in these particular ethnic groups.

The figures may represent a statistical blip or they may say something profound about different responses of the immune system to the virus or the vaccine which could help the hunt for prevention, treatment and even a cure. If the trials had not been done, the data would not exist. Even in failure, VaxGen has advanced the search for a vaccine.

There are quite a number of other potential vaccines, many based on different approaches, in the pipeline. One of the most promising is a collaboration between the Universities of Oxford and Nairobi and is based on the observation that some Kenyan prostitutes who regularly had sex with HIV-positive truck drivers somehow did not themselves become infected. There may be a lot more failure in the years ahead, but scientists are convinced that it will be possible in the end to find a vaccine against HIV. IAVI says the world must put in a lot more money to make it happen and happen sooner rather than later. There could hardly be a more important scientific cause.