They have also topped their own donation of $3.3bn last year split between the William H Gates foundation and the separate Gates Learning Foundation, which has assets of $.3bn. The record donation was disclosed, as the foundation announced the award of a $50m grant to a Seattle-based team aiming to develop a new vaccine for malaria. Other grants from the foundation recently have included $50m to Columbia Universityfor a programme to reduce maternal mortality in developing countries. The couple have also given $25m to an international team aiming to develop an effective vaccine to prevent HIV infection. Bill Gates senior, director of the William H Gates Foundation, said: "Bill and Melinda are enthusiastic about the opportunity to improve the lives of people throughout the world. " The malaria grant is being given to the Program for Appropriate Technology in Health (PATH), a non-profit organisation under whose umbrella a group of international health experts has been assembled. Its president Gordon Perkin said that a child is dying of malaria every 20 seconds.

"Malaria is exacting an enormous toll on the health and economic well being of hundreds of millions of families, most of who live in developing countries," he said. "Although drugs that treat the illness have been around for some time, the malaria parasite continues to develop resistance to these drugs. The world badly needs a vaccine."