The Thai participants, the first of an expected 2,500 people to join the programme, received the jabs at a Bangkok medical centre on Wednesday.

Most of the volunteers are young male intravenous drug-users who are at high risk of HIV infection.

The vaccine being tested is produced by the California-based company VaxGen Incorporated, which also started the world's first trial of the vaccine in the US last June with a planned 5,000 volunteers.

Although about 30 possible Aids vaccines are being developed, VaxGen's is the only product which has reached so-called Phase III testing, which involves large-scale tests on human subjects.

Different vaccine

The vaccine being tested in Thailand is different from the product used in the US in order to match the different strains of HIV which prevail there.

The Thai trials, approved by the Thai ministry of health, are expected to cost between $6-9m.

They are being financed by VaxGen and some other US bodies with no Thai public money involved.

Preliminary results on the effectiveness of the vaccine will be available in 30 months, with the full study taking three or four years to complete.

Jose Esparza, the UN Aids vaccine team leader said: "We need to test different types of vaccines because we don't know which ones will work."

He described the trial as "very complex" logistically and ethically.

Foundation for future

Half the volunteers will get a placebo while the other half receive the actual vaccine. Participants will not know which group they fall into.

The experiment will help build the foundation for future clinical trials on vaccines in the developing world, UN Aids said.

Volunteers will get injections of vaccine or placebo

Other developing countries where Aids vaccine tests have been conducted include Cuba, Brazil and China.

Altogether some 30 different types of vaccines have been tested since 1987, mainly in the US, in phase I and II trials.

Some 16,000 people, mainly in the Third World, become infected with HIV every day.

Each year some $200m is spent on Aids vaccine research globally. Mr Esparza said this amount was "too low".