Project director Dr Supachai Rerks-Ngarm said the organisers aimed to rope in 16,000 people but that so far about 5000 of 7000 volunteers had passed the screening process.
"We thought the recruitment process would only take one year, but we need more time so we will have to extend for another year," he told AFP.
Supachai said participants had to meet criteria including that they be aged between 20 and 30, in good health and HIV-negative.
Many prospective candidates were rejected because they did not meet the standards.
"We have established a strict screening process to maintain maximum standards in carrying out the trial," he told AFP, adding that about 3000 people have already received the vaccine.
The recruitment process was initially scheduled to run for one year from September 2003, with the results of the study due five years after the start of the project in early 2004.
The trial is the first to determine the efficacy of an HIV vaccine, in a so-called "third phase" test carried out after the first and second phases ensured it was safe for humans.
A phase three trial involves large-scale testing on ordinary citizens in a real-world situation, and is the last hurdle before the vaccine can be registered and administered freely.
The project is being held in the two eastern provinces of Chonburi and Rayong, where 47 screening centres and eight vaccine injection centres have been established.
"We thought the recruitment process would only take one year, but we need more time so we will have to extend for another year"
Dr Supachai Rerks-Ngarm,
A combination of two vaccines developed by Aventis Pasteur and VaxGen will be administered over six months in six injections, and volunteers' progress will be monitored every six months for three years.
If successful, the therapy would give life-long protection against the HIV virus that causes AIDS.
The trial is a collaborative effort among the United States Military HIV Research Program, the Royal Thai Army Medical Department and the US National Institute of Allergy and Infectious Diseases.
Thailand is to host the World Aids Conference from 11 to 16 July, in recognition of the growing crisis faced by the Asia Pacific region where infection rates are rising rapidly in many nations.