The International Aids Vaccine Initiative (IAVI) has pumped more than $9m into the projects.

One is a joint venture between UK and Kenya scientists, the other a partnership between scientists from the US and South Africa.

The aim will be not only to develop a vaccine, but also to ensure that it is made available in the worst affected areas of the world.

Latest figures show that the number of people infected with the HIV has jumped by 10% during 1998 to 33.4 million. The worst affected area is sub-Saharan Africa.

Dr Seth Berkley and Dr Anzal discuss the bid for an international Aids vaccine on Radio 4's Today programme

Dr Seth Berkley, president of the IAVI, said: "Our goal is not only to ensure the development of an Aids vaccine as soon as possible. Our goal is to make certain it is available to anybody in the world who needs it."

The UK-Kenyan team will develop a HIV vaccine that combines two separate vaccine components.

They will use a DNA vaccine carrying a small part of the gene for HIV to stimulate an immune response in the body's t-cells.

These white blood cells attack HIV infected cells and kill them before they can start reproducing the HIV virus.

Vaccines are under development

The DNA vaccine will be combined with a modified vaccinia Ankara (MVA) virus vaccine. The MVA vaccine, previously used to combat smallpox, helps to amplify the body's immune response, making it approximately 100 times stronger.

The researchers hope the combination will prove to be an effective way to fight HIV infection.

Professor Andrew McMichael, of the Medical Research Council Human Immunology Unit at Oxford University, said: "We are relatively well advanced. We know from animal studies that the vaccine stimulates the right kind of immune response. We now have to see if it works in humans."

However, he said it could still be at least seven years before the vaccine is widely available.

He said three phases of tests were needed first to establish the vaccine's efficacy and safety.

Parallel approach

The US-South African team will work on the development a separate virus based on VEE alphavirus replicon particles, again designed to stimulate the body's immune system.

Professor McMichael said it was important that different vaccines were developed.

Not only might only one vaccine prove successful, the use of different carrier viruses for the vaccines would reduce the risk that the immune system could become used to repeated doses of a single vaccine, and cease to respond in the desired way.

Clare Short, UK Secretary of State for International Development, welcomed the new initiatives.

She said: "During the course of one day about 16,000 people will be infected with HIV.

"A preventative Aids vaccine must be produced and made available to developing countries."