The one thing that would really change the picture would be a vaccine

Bill Gates

The donation is intended to fund a four-year project taking a number of vaccines from the development stage to human trials by the International Aids Vaccine Initiative (IAVI).

At the summit, riot police have thwarted an attempt by several hundred anti-capitalist demonstrators to disrupt the meeting.

Police used water cannon and moved forward with a line of armoured vehicles, pushing the crowd away from the Congress Centre where the meeting was taking place. There were no reports of any arrests.

'Unbelievable failure'

Mr Gates said that donations like his were meant to correct the "unbelievable market failure" of not developing a vaccine 20 years after the first Aids cases emerged.

Police and anti-globalisation demonstrators engaged in a tense stand off in Davos

"The one thing that would really change the picture would be a vaccine," Mr Gates said, conceding that "there's no way commercial companies are going to see this as a priority".

The Yahoo internet company responded by promising $5m to an Aids awareness campaign.

"Aids is a devastating global problem and we look forward to lending the power of our global network to help find a solution," Yahoo co-founder David Filo said in a statement.

Vaccine trials

Trials on a new vaccine will begin in Kenya in the next few days according to Seth Berkley, head of the IAVI.

Mr Berkley told reporters at the Davos meeting that the vaccine, which has been tested since August on volunteers in Oxford, UK.

This experimental vaccine was developed after doctors found that a group of prostitutes in Nairobi never contracted HIV, despite repeated exposure to infection.

It is one of around 25 vaccines being tested on humans around the world but is the first to target the "A" strain of the HIV virus prevalent in sub-Saharan Africa.

In all, the IAVI is seeking $550 million to fund its development work through to 2007.