BANGKOK (Reuters) - Thailand will hold the world's biggest HIV ( news - web sites) vaccine trial, lasting five years and involving 16,000 people, a government health official said on Friday.

Volunteers aged between 20 and 30 will be selected from the general population rather than high-risk groups such as drug users.

They will take a combination of two vaccines, called the "prime boost strategy," over a period of six months as they carry on their usual activities.

"Thailand sees 20,000 to 25,000 new cases of HIV infection every year. We really want to reduce this number," said Prayura Kunafol, an advisor to the Department of Communicable Disease Control.

The trial will be officially launched on Monday at the 14th International AIDS ( news - web sites) Conference in Barcelona.

Prayura said the trial should begin by the end of the year in the eastern provinces of Chonburi and Rayong, where four to six in every 1,000 people contract HIV annually.

He said Thailand was ready to launch the trial but needed final approval from international and Thai regulatory bodies. The vaccine has already been tested for safety and this large-scale trial is the final stage. If successful, the vaccine could be licensed and would give life-long protection against the virus that causes AIDS, he said.

Prayura said a survey had shown enough people would be willing to take part in the community-based trial.

More than 70 million people will die of AIDS in the next 20 years and 40 million people are currently infected, a United Nations ( news - web sites) report said on Wednesday.

The trial is a collaborative effort between the Thai government and the U.S. Military HIV Research Program.

Other partners include French drug maker Aventis Pasteur and U.S. biotech firm VaxGen Inc. who helped develop the vaccine, Prayura said.