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

They will take part in a phase III trial, the United Nations Aids programme said on Tuesday.

The announcement comes a day after the launch of Africa's first Aids vaccine trial in Uganda which involves 40 people.

American beginnings

The initiative, due to begin in two to three weeks, follows a phase III vaccine trial begun in the US in June 1998. The organisers recently called for more volunteers to allow the trial to move towards completion.

Most of the US volunteers were at risk of HIV infection, which leads to Aids, through sexual transmission.

The Thai trials, approved by the Thai ministry of health, are being conducted by the California-based firm VaxGen, said Jose Esparza, the UNAids vaccine team leader.

The vaccine to be tested in Thailand - which has undertaken a series of smaller experiments - will differ from the product used in the US in order to match the various strains of HIV which prevail there, Mr Esparza said.

The Thai experiment is estimated to cost between $6-9m. It is being financed by VaxGen and some other US bodies.

There is no Thai public money involved, Mr Esparza said.

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

Mr Esparza said: "We need to test different types of vaccines because we don't know which ones will work."

Ethical concerns

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

Volunteers will get injections of vaccine or placebo

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, UNAids said.

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".