Steve Connor: Money is not the answer to this problem

Vaccines were one of the great medical success stories of the 20th century – the eradication of smallpox and vanquishing of polio and measles are testament to their success. But an HIV vaccine poses an altogether different challenge, largely because of the unique characteristics of this elusive virus.

The failure of the Merck clinical trial of the most promising HIV vaccine at the end of last year is widely seen as a major setback, and many Aids scientists now believe that a radical change in the direction of research is now needed.

This is not just about throwing money at the problem, as Gordon Brown suggested in 2005 when, as Chancellor, he launched the International Finance Facility to boost funding on HIV vaccine research, among other projects aimed at improving the health of the developing world. Mr Brown lamented that, worldwide, only £400m a year was spent on researching an HIV vaccine, which was projected to lead to a partly effective vaccine by 2020. "If, by doubling research and development spending over the coming five to 10 years we could bring forward the discovery of an Aids vaccine, we could save millions of lives," he said.

Unfortunately science doesn't work like that. Unlike an engineering project such as the Channel tunnel, throwing twice as much money at a problem does not mean you get there twice as quick.

Trying to develop an HIV vaccine is a bit like digging a hole in the ground, but unlike the Channel tunnel you don't know where the digging might lead. The question is whether at some point we should stop. Few of the scientists who took part in our survey believe that this time has come, and most would no doubt say that an HIV vaccine is too important to ever give up on.

For the past quarter of a century, scientists have made tremendous advances in analysing and understanding the uniquely difficult properties of the virus that causes Aids. But an HIV vaccine is as far away as ever.

However, it took nearly 50 years to develop the polio vaccine and nearly as long to make an effective vaccine against the measles virus. And as any malaria scientist will tell you, the history of this pernicious disease is littered with failed vaccine attempts.

So the time to give up on an HIV vaccine has not yet come, and perhaps it never will, given that prevention based on education does not appear to have stopped the spread of the virus.

But the reality is that an HIV vaccine is one of the most difficult scientific problems of our age. Scientists need our support and encouragement to redouble their efforts so that one day the scourge of Aids will be history – just like smallpox.

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