Volunteers in Philadelphia, who are considered to be at high risk of contracting the virus that causes Aids, received their first inoculations on Tuesday.

The volunteers said they wanted to show the world that there was hope for a vaccine capable of overcoming the threat of Aids.

VaxGen, the US company that developed the vaccine known as Aidsvax, will be testing it on 5,000 volunteers in America and 2,500 more in Thailand over the next three years.

Although dozens of potential Aids vaccines have been developed in the last two decades, none have been extensively tested on human beings.

Clinical trials are usually the last step before seeking the approval of the Food and Drug Administration to market a drug or vaccine.

Analyzing the results

Each volunteer will receive seven inoculations during a 30-month period. Some will receive the vaccine and others a placebo, an inactive substance used in controlled experiments to determine the efficacy of a vaccine.

Neither researchers nor volunteers will know which people are given the vaccine.

But at the end of the trial, the rate of infections in the group receiving the placebo will be compared to the infection rate among volunteers who received the vaccine.

How the vaccine works

Aidsvax is based on the proteins found on the coating of the HIV virus. It is intended to stimulate the body to create antibodies that would stop the virus from attaching to receptors on the white blood cells it attacks.

However, Aidsvax does not use a live strain of the HIV virus, so volunteers should not be at risk of contracting Aids from the vaccine.

But many experts in the field are sceptical, believing that antibodies alone, even derived from live strains of the virus, will not provide an effective vaccine.

Potential vaccines derived from DNA, the genetic code of the virus, are now being worked on but will not be available for trials on humans for some time.

'Vaccine only answer'

Experts agree that, in the long term, a vaccine is the only answer to HIV.

Recent research by a group of American scientists suggests that the HIV virus is capable of resisting all the drugs approved to treat the disease in the United States.