OUAGADOUGOU, Dec 13 (AFP) - The search for a vaccine to combat Africa's AIDS pandemic is being clouded by a focus on viral sub-types that predominate in western countries and the worrying ability of the virus itself to mutate, scientists said Thursday.
Of the 40 million people in the world with the human immunodeficiency virus (HIV), more than two-thirds live in sub-Saharan Africa, which is home to 10 percent of the world's population.
The predominant sub-types of HIV-1 -- by far the most prevalent and vicious of the two known major strains of the virus -- are A in eastern Africa; C in southern Africa and the Horn of Africa; and a mixed one, A/G, in western and central Africa.
But of the number of vaccines being trialled today, 70 percent are modelled on the genetic profile of sub-type B, which predominates among HIV patients in the United States.
Only three percent are for sub-types that are cutting a swathe through Africa.
"The truth is that we do not know whether a vaccine designed for one kind of sub-type will work on another, or if so what its effectiveness will be," Eric Delaporte, a virologist at France's Research Institute for Development (IRD) told AFP.
"In vitro, it appears to work, but there has been no confirmation in vivo, and of course we still do not know about the results on humans," he said. In vitro means using antibodies of sub-types in laboratory vessels; in vivo means using lab animals, which in this case is the monkey.
Even though AIDS was identified 20 years ago, there is still no vaccine for it. Only one candidate vaccine, called gp120, has made it through to Phase III trials, the final step in the long and arduous process to see if it is safe and effective for humans.
"HIV vaccines should be designed for effectiveness in sub-Saharan Africa," said Max Essex, a Harvard University vaccine expert who is working in Botswana, the worst-hit country in the worldwide AIDS pandemic.
"Any vaccine that works must be made available first, or concurrently, in sub-Saharan Africa," he said in a presentation at a regional AIDS conference in Ouagadougou, the capital of Burkina Faso.
Adding to the problem is the remarkable ability of viral sub-types to promiscuously swap their genes, fuelling the fear that HIV could become a shifting target for vaccine engineers.
"Recombinant viruses contribute to 10-30 percent of the global pandemic," said Soleymane M'Boup of Senegal, who discovered the other major strain of HIV, called HIV-2. "The frequency of subtypes will increase."
The gp120 vaccine, called AIDSVAX, uses a portion of the outer surface, or envelope, of the HIV in the hope of stimulating an immune response.
It is being tested on volunteers in the United States and Thailand; the results will be published some time next year, but most AIDS experts caution against hopes that it will be a silver bullet like the polio or smallpox vaccines of legend.
At best, it will give only partial protection and will have to be followed up with improvements, they say.
Another widely explored technique entails genetically engineering relatively harmless replicating viruses to produce some HIV proteins that, it is hoped, will then stimulate HIV-specific immune responses.
One of these candidates, based on canarypox, is in Phase II trials.
Other methods, all of which are only in Phase I trials, entail using harmless bacteria as a Trojan horse to transport HIV genetic material into the body, thereby provoking an immune reaction; and so-called "naked DNA" vaccines in which the actual genes of HIV are introduced into the body, where they are taken up by cells which then produce proteins that stimulate the immune system.
Delaporte said that once an effective vaccine was found, it should be possible to deal with new recombinants in the same way that scientists are able to tweak flu vaccines to deal with new sub-strains that pop up every year or so.
"But this requires good epidemiological infrastructure," he said, noting that it was vital to find out on the ground which new recombinant strains were emerging in order to adjust the vaccine accordingly.
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