It will include 40 people and uses a vaccine made by Pasteur Merieux Connaught, a division of France's Rhone-poulenc Group.

The phase 1 trial aims to show only the safety of the vaccine. Later stages will look at the effectiveness of the preparation.

Advances over the last 10 years mean that many drugs are available to help combat Aids in Europe and the US.

However, because of their cost they are generally unavailable in parts of Africa where the disease is most prevalent.

First step

Dr Jerrold Ellner, of Case Western Reserve University in Ohio, is helping direct the study.

He said: "Although small in size, this trial is important symbolically as a first critical step in developing an effective vaccine for Africa."

Up to a quarter of adults in some African nations are infected with the HIV virus, with more than 22 million people in all of sub-Saharan Africa infected.

While more than 45 vaccine trials were underway in volunteers in the United States, this is the first in Africa.

Dr Anthony Fauci is director of the US National Institute for Allergies and Infectious Diseases (NIAID), which is sponsoring the trial.

He called it "an important first step in an attempt to develop HIV vaccines for countries hardest hit by the Aids pandemic".

Canarypox

The vaccine being used is called ALVAC vCP205. It is one of several vaccines being tested that are based on the canarypox virus, which does not cause disease in humans.

The canarypox virus had been genetically engineered so that it contains three HIV genes.

The idea is to prime the immune system to recognise and attack anything carrying the relevant markers.

Such vaccines work against other viruses. But HIV is especially hard to vaccinate against, as it strikes the immune cells stimulated in a vaccine.

Researchers had hoped a live but genetically weakened version of HIV would be more potent.

But recent tests in monkeys show that even a crippled virus can reconstitute itself in the body and cause infection.

Methods

In the trial, 40 healthy HIV-negative adults between 18 and 40 will be recruited.

They must be considered at low risk of becoming infected and will be counselled about ways to avoid HIV infection.

Half will get the vaccine, 10 will get a similar canarypox-based vaccine for rabies and 10 will get dummy injections.

Each will get four injections over six months. The researchers will then examine their blood for signs of an immune response.

Of 20 million people in Uganda, half a million have died of Aids, according to NIAID. Four to ten per cent of Uganda's rural population has HIV.

The figure for city dwellers is up to 25%.

The vaccine has already been shown to be safe in nearly 1,000 volunteers in US and French studies.

The start of the trial was delayed last summer, partly due to the death of a senior Ugandan Aids researcher.