The vaccine, developed by scientists working at the Medical Research Council in London, works by stimulating the body's own defences against the Aids virus.

The trial of the new vaccine began two years ago when 30 volunteers, who were not at risk of Aids themselves, were given the vaccine in three separate injections over a period of six months.

They developed a powerful antibody response, showing that, as the researchers expected, the vaccine was stimulating the body's natural defences.

What pleased the scientists was that the response was ten times more powerful than had been achieved with other experimental vaccines.

That is because the vaccine had been mixed with certain chemicals designed to make it more potent.

When the vaccine was tried in animals, it protected them against infection with an Aids-type virus.

Now, doctors are hoping to mount a large-scale trial involving thousands of people at risk - perhaps in Africa.

Prof Weber: More optimistic about a vaccine

Professor John Weber of Imperial College School of Medicine said: "I'm much more optimistic about the possibility of a vaccine than I used to be, for a number of reasons.

"We understand what type of immune response we're going to need to generate to protect people against HIV infection. Secondly, we know better how to generate these immune responses with novel types of vaccine. And thirdly, we have now developed new chemicals which make more powerful immune responses."

Since the advent of Aids in 1981 research has been swift. In 1984, the virus which causes Aids was identified and two years later the drug AZT became the first of several which kept Aids at bay.

But in 1992 there was disappointment for researchers when the virus began to show resistance to the drugs.

Combinations of drugs can help keep Aids at bay

Two years ago combinations of drugs were shown to work well and last year types of drugs known as protease inhibitors were found to be highly effective.

Researchers now say the challenge is to find out what combinations of drugs work and in what quantities.

Dr Peto: challenge to find correct drugs combination

Dr Tim Peto of the Medical Research Council said: "The challenge is to do trials on different mixtures and find out how to give them, what order to give them in and how many to give at once."

In Britain, death rates from Aids are 30% down. There is still a long way to go but doctors believe Aids is at last showing promise of becoming a more manageable illness.