There is no vaccine against AIDS and none of the dozens of vaccines being tested is likely to completely protect people from the deadly virus, but the future looks bright for AIDS vaccine development, researchers said on Tuesday.

Scientists will learn from the vaccines now being tested, and the developing world, hardest hit by HIV, is starting to produce its own vaccine effort, said Dr. Seth Berkley, head of the nonprofit International AIDS Vaccine Initiative.

“An AIDS vaccine is the only tool that can end the pandemic,” Berkley told a news conference to launch a biennial report on vaccines at the 16th International AIDS Conference in Toronto.

“All evidence suggests that a vaccine is possible. There is progress being made. It’s slow but it’s steady,” he added. “To me, we are about to enter a renaissance in AIDS vaccine research.”

Yet Berkley said only two AIDS vaccine candidates are in advanced human trials - one made by Merck and Co. and another by Sanofi-Aventis SA.

“The next major milestone for the field is likely to be the Merck result, which is a test for cellular immunity,” Berkley said.

Berkley and others do not expect the Merck vaccine will protect against disease in the way, for instance, a measles vaccine does.

The AIDS virus infects more than 39 million people globally, more than 60 percent of them in sub-Saharan Africa. It kills more than 4 million people every year and has killed 25 million people since it was identified in the 1980s.

It is difficult to vaccinate against because the virus infects the very immune system cells that are usually stimulated by a vaccine.

“This is probably the toughest adversary that has ever been out there for vaccine development,” Berkley said.

Most vaccines stimulate antibodies, immune system proteins that mark enemy invaders for destruction. Vaccine researchers believe that a good AIDS vaccine will have to stimulate both antibodies and so-called cellular immunity, which is the job of T-cells, dendritic cells and other immune cells.

Berkley hopes results of how well the Merck vaccine works will be available in 2008. If it reduces infection rates by even a little, scientists can study the volunteers and see just what successful aspects could be used as clues for further research.

If it does not work at all, whole new approaches will have to be pursued.

Even a partially effective vaccine could be useful, said Stephen Lewis, the United Nations delegate to Africa for AIDS.

“I think it’s fair to say ... even a modestly effective vaccine could cut the number of new infections by one-third over a decade, saving tens of millions of lives,” Lewis told the news conference.

One obstacle to testing vaccines is a lack of volunteers and facilities to do the clinical trials, the tests in people that show whether a vaccine or drugs works.

“We have dramatically improved the ability to do clinical trials,” Berkley said. Ten years ago such trials were only done in industrialized countries, but now they are being run in two dozen developing countries, he said.

And funding for trials has doubled over the past five years, he said.