The BBC's Jennifer Jensen on the Aids vaccine trials

Troy Masters is taking part in Phase III trials for AIDSVAX, a possible vaccine for Aids.

He has a personal reason for being involved in the trials. Fifteen of his friends have died of Aids in the last decade.

He says he is willing to put up with the side effects caused by AIDSVAX.

"The risks of not doing this are greater," he said. "Living with HIV in the world is a much greater risk than suffering a rash on my arm or a sore arm."

Proteins

AIDSVAX is manufactured by VaxGen and is being trialed in the USA and Thailand.

Trials began in the USA soon after approval for the Phase III trial in June.

They began in Chicago, Denver, Los Angeles, Philadelphia and St Louis and at the end of the year will cover the whole country.

AIDSVAX is to be tested on 5,000 Americans and 2,500 Thais

The vaccine is made from proteins on the surface of two different strains of HIV.

Because it is not made from live virus, VaxGen says there is no chance of it causing people to get HIV.

The company is looking for 5,000 male and female volunteers to take part in the US trials. They must be aged 18 to 60, not have HIV but be at risk of contracting it through sexual contact.

Injecting drug users are not taking part.

Two thirds of the volunteers will get the vaccine and the other third will get a placebo.

They will receive the vaccine seven times over a three-year period.

Volunteers will use diary cards to record any side effects, but VaxGen says so far the only reaction has been a sore arm.

Waste of time

The volunteers will also be assessed for risk-taking behaviour and tested every six months for HIV.

Some, however, have questioned the scientific value of the vaccine trials. They say VaxGen is only interested in making profits.

Gregg Gonsalves of the Treatment Action Group, said: "It is a waste of time, money and the good will of volunteers around the country.

"The company has been developing this product for 10 yeras and it wants to get it on the market.

Gregg Gonsalves says AIDSVAX is 'a waste of time'

"The scientific community is not impressed, but the company has a mission to get the product out as quickly as possible."

Scientists say the first version of AIDSVAX did not cover all strains of HIV circulating in the population, but the vaccine has since been improved and now has a broader representation of HIV strains.

The International Association of Physicians in Aids care says only a clinical trial will determine the effectiveness of the new vaccine.