The first Aids vaccine to be tested on humans has produced disappointing results.

The vaccine, produced by American biotech company VaxGen, does not protect the general population, but may protect blacks and Asians.

What is the significance of this announcement?

If results had shown the vaccine to be 30% effective, it could have been licensed for general use within five years.

However, the fact that the protection rate was much lower than this means that the whole process is likely to take significantly longer.

Are other vaccines in the pipeline?

The VaxGen vaccine, known as Aidsvax, is the first to complete final Phase III trials on human volunteers.

The company is also conducting a test of 2,500 intravenous drug-users in Thailand, with results to be released later this year.

Several other potential vaccines are at an earlier stage of development

How do Aids vaccines work?

These HIV vaccines prevent infection by stimulating the production of antibodies.

Aids vaccines tend to use specific parts of HIV, such as genes or proteins to activate the body's immune defences.

For instance, the VaxGen vaccine makes use of a specific protein on the surface of HIV which enables the virus to dock with an immune-system cell, the first stage before penetrating it.

The hope is that once the body recognises these fragments it will mount a vigorous defence which will help when it encounters the real virus.

To combat HIV effectively, scientists will have to follow the model of the flu virus and constantly change the vaccine to try to keep up with the latest mutations of the virus.

Why has it taken so long for scientists to reach this stage?

It is 21 years since scientists began the battle against Aids, but it is an extremely complicated condition.

There are different strains of HIV, and the virus constantly mutates, meaning scientists have found it very difficult understand how it works.

We are seeing developments in treatments and vaccines now because experts understand much more about the virus.

This is partly because they have been able to monitor people living with HIV and see how they respond to antiretroviral medications.

Who was the vaccine tested on?

The VaxGen vaccine was tested on high risk groups. In North America and Europe, it is being tested on gay men, and in South East Asia, it is being tested on drug users.

Who will be the first to receive an effective vaccine?

Seth Berkley, president of the International Aids Vaccine Initiative, told the BBC that once a vaccine was available, it would be directed first of all at high risk groups, such as commercial sex workers and intravenous drug users.

But he said once it had been proved to be safe and effective it could be made part of the immunisation programme. "We would hope to protect every child form this disease."

So will everyone who needs it be able to have the vaccine?

There are both scientific and practical problems in ensuring everyone has access to a vaccine.

Different stains of HIV tend to be prevalent in different areas.

As the Vaxgen vaccine is being tested in Europe, Asia and America, so scientists do not yet know if it will be of any help to people in sub-Saharan Africa, currently hardest hit by the Aids epidemic, where a different strain predominates.

Campaigners are also calling for governments to ensure the infrastructure is in place so that once a vaccine is developed it can be manufactured and distributed rapidly, and at a cost which developing countries can afford.