The scientific quest for an AIDS vaccine has received a $1.5 million boost from the William H. Gates foundation, which bears the name of Microsoft's main man.

The contribution to the International AIDS Vaccine Initiative (IAVI) was made on Sunday, and brings the development fund to $15 million, officials said.

AIDS experts, however, have said the total cost for a workable vaccine could go much higher -- perhaps into the billions.

Before an effective vaccine can be developed, scientists must find what to target to control HIV, a virus which is constantly changing and evolving.

Several promising vaccines have failed in early human trials.

One vaccine candidate is currently being tested in the largest human trial to date.