"Twenty years after the human immunodeficiency virus (HIV) was identified, the AIDS pandemic is one of the greatest challenges to public health on a global scale.

"The need for a vaccine against HIV is thus greater than ever."

Antibodies are the molecular frontline forces in the body's defence system, designed to tag an invading virus or bacterium so that it is destroyed by immune cells.

Many vaccines, such as those against the flu, polio and measles, are based on antibodies, using a piece of the virus to prime the immune system so that it identifies the intruder in the future.

But antibody designs against HIV have so far been a huge disappointment. So far, none has been found that can deliver a big immune punch.

One suspected reason is that wild types of HIV may be somewhat different from the strains used in labs.

Among wild strains, the proteins on the surface of the virus which are often used as the antibody primers may be more difficult to detect because they are folded over and may be camouflaged by slippery sugar molecules.

Another likely reason for the failure is viral variety -- with different sub-types of HIV and mutation, the antibodies may be hunting an absent or a shifting target.

The French team has taken a different and more specific tack, targeting a tiny area of a surface protein that, they say confidently, is common across the range of HIV types.

This area is called CBD1 and is part of the gp41 protein. CBD1 binds to a protein in the T-lymphocyte immune cell called caveolin-1, thus helping the AIDS virus to dock to and infiltrate its target.

The researchers synthesised a chain of peptides -- the building blocks of proteins -- corresponding to CBD1 and immunised rabbits with it.

In lab-dish experiments in which blood taken from the immunised rabbits was exposed to human T-cells and the virus, it proved to be a remarkable shield against a range of sub-types of HIV-1, the commoner and more vicious form of the two big strains of the virus.

"The anti-CBD1 antibodies work in two ways," the press statement said. "Firstly, they inhibit cellular infection by HIV, and secondly, among cells that are already infected, they lead to the production of defective viruses (which) are unable to infect other cells."

The vaccine is only experimental and has not been applied to any human volunteers to see whether it is safe or effective.

Even so, the findings are deeply encouraging, the French scientists say.

"It shows that the CBD1 area can be used as a specific target for developing an effective immune response."

The study, lead-authored by Ara Hovanessian of the Pasteur Institute and National Centre for Scientific Research (CNRS), was published on Tuesday in a specialist journal, Immunity.

More than 20 million people have died of AIDS and some 38 million more have HIV, the virus which causes it.

But in the entire 23-year history of AIDS, only one vaccine, a design based on the gp120 protein, has ever gone through the entire three-phase test process, and it proved a disappointing flop.

The Hyderabad-based company withdrew all six of its drugs submitted to the WHO's pre-qualification listing, which sets minimum quality standards for cheaper generic anti-AIDS medication, WHO spokesman Iain Simpson said.

The move followed systematic WHO inspections at the outside laboratories contracted by the drug makers to carry out the testing, he told journalists.

The pre-qualification scheme is a cornerstone of attempts to supply more affordable anti-AIDS drugs in poor countries where they are most needed by governments and aid agencies.

Ten other generic anti retrovirals made by the Indian drugs giant Ranbaxy have already been struck off the list as well as two more made by Cipla -- also an Indian drugs company -- earlier this year.

All have suffered from problems with the independent laboratory tests which are meant to show that they are as effective as the original brand-name medicine they copy, a process known as "bioequivalence", according to WHO.

"It's become clear that some of the data gathered to show the bioequivalence of these generic drugs produced by Hetero was not satisfactory," Simpson said.

"The way they have done their analysis does not actually show that. These products may be bioequivalent, we just don't know at this point," he added.

The withdrawal leaves 48 approved anti retrovirals on the WHO prequalification list.

The aid group Medecins Sans Frontieres (Doctors Without Borders - MSF), which runs several treatment programmes for HIV/AIDS patients in developing countries, expressed concern about the problems.

"We are disappointed by the behaviour of Hetero and Ranbaxy, that have submitted paperwork that was not valid," MSF director Daniel Berman of MSF told AFP.

Berman said MSF had used drugs by Ranbaxy. All its programmes had been peer-reviewed and clinical results showed high survival rates for impoverished AIDS victims on generic anti-retrovirals, he added.

The Indian companies were resubmitting their drugs with new tests, and they were expected to be approved early next year, the WHO said.

Simpson insisted that the drugs had already passed safety and manufacturing quality tests under the scheme.

"At that stage it is clear that these are quality medicines, they are not dangerous medicines," he said.

However, the doubts surrounding the methods used by the sub-contracted laboratories have prompted the health agency to consider expanding its systematic inspections beyond India.

"There are certainly major concerns about the quality of the lab testing and the situation that has been found by these inspections," Simpson said.

But he claimed the results showed that the checks put in place by the international pre-qualification system were working.

"They are effectively helping the generic manufacturing industry to ensure the quality of its products."

Berman said the findings were also prompting wealthy countries in Europe and North America to make closer checks on outside testing.

The single international approval for generic and brand-name drugs was approved by WHO member states earlier this year, despite fears that the scheme could shortcut stricter national regulatory approval.

It followed controversy over attempts by poor countries and aid agencies to seek cheaper generic copies of costly, branded anti-retroviral drugs to treat millions of HIV/AIDS patients.

Pharmaceutical giants feared that their use could undermine the original patented brand-name drugs and leave research costs uncovered.