PARIS, Nov 30 (AFP) - Medical researchers, grass-roots activists and patients will mark World AIDS Day on Wednesday amid frustration and not a little disappointment in the battle against the last scourge of the 20th century.

According to the latest UN figures, more than 16 million people have died since acquired immune deficiency syndrome (AIDS) manifested itself in the late 1970s, and 33.6 million have HIV, the virus that leads to AIDS.

But the fight against the accelerating epidemic has been marked by painfully slow progress, and sometimes setbacks.

About two dozen vaccines have been tested, usually crafted from proteins found on the surface of the AIDS virus and delivered by another, inactivated virus.

The vaccines have generally succeeded in triggering antibodies -- but not in sufficient numbers to destroy cells that become infected by HIV, a powerful and changing virus.

The news is somewhat better when it comes to treatment for those who have already contracted AIDS.

Among an array of interesting new drugs that have emerged in the past few years are antivirals, which can prevent transmission of HIV to new-born infants if they are administered to the pregnant mother.

Then there is a combination cocktail of antiviral drugs, which unleashed a surge of optimism by keeping HIV below detectable levels in many patients.

But then a hammer blow fell. Researchers reported that combination treatment can suppress but not elimate HIV.

According to Anthony Fauci, of the US National Institutes of Health, the virus appears to hide in the body, only to return to the attack as soon as treatment stops. This implies that the cocktail -- a gruelling daily regimen -- may have to be taken for the rest of one's life.

More, the high cost of drugs cocktail means that it is available to only one in 10 of the world's HIV population. To live in a poor country ravaged by AIDS is to be denied the latest treatment or perhaps any treatment at all.

The prestigious British medical journal The Lancet last week took aim at the United States for exerting trade pressure on developing countries that limited access to affordable treatment for patients with HIV and AIDS.

It gave the example of Thailand, where one million out of a population of 61 million have HIV -- and where a course of combination antivirals costs 675 dollars per month, compared with the typical monthly wage of an office worker of 110 dollars.

Local firms could easily make generic drugs "at a fraction of the cost" of imported US medicine, but they have been thwarted by US pressure on patent protection, The Lancet said.

"It remains to be seen whether WHO (World Health Organisation) and non-governmental organisations will be able to prevent western trade pressure from forcing less-developed countries to forego right to produce and import medicines that are prohibitively expensive in today's markets," The Lancet said.

As a cure seems to be over the horizon, efforts are being focussed on more immediate problems -- preventing the disease from spreading and tackling the social and economic consequences of AIDS.

That impact will be felt most keenly in Africa, the continent that has the least financial and educational resources.

Around two-thirds of the people with HIV in the world live in sub-Saharan Africa. In the southern countries of the continent, life expectancy from birth could drop from 59 (in the early 1990s) to 45 within a decade -- almost the same figure as in the early 1950s.

In these countries, governments are grappling with the mounting tragedy of how to look after children left orphaned by age, while more and more companies are worried about how they will cope with the loss of skilled workers to the disease.

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