WASHINGTON, Sept 7 (AFP) - A number of new AIDS vaccines tested on primates have shown success in blocking development of HIV, the virus that causes AIDS, according to studies presented this week at the biennial AIDS vaccine conference in Philadelphia.

Whether the results will transfer to humans has yet to be determined, however, warned organizers at the "AIDS Vaccine 2001" conference.

"Perhaps the greatest obstacle to HIV vaccine development is an insufficient understanding of the correlates of immune protection, which are better understood for other viral diseases," said Anthony Fauci, director the National Institute of Allergy d Infectious Diseases, part of the National Institutes of Health.

Vaccine tests on monkeys, whose immuno-defenses were stimulated to prevent proliferation of the virus, in a number of cases reduced HIV to low or undetectable levels. Tests more than a year later found no change in the positive results.

Although vaccines in trials now cannot immunize against the Human Immunodeficiency Virus itself, they appear to be effective in dramatically reducing HIV mortality, experts proclaimed.

Yale University researchers, in presenting their findings Friday to also appear in the US journal Cell, explained how they injected seven rhesus monkeys with a weakened form of the vesicular stomatitis virus (VSV) modified with HIV, then infected them with HIV.

Eight other monkeys were infected with HIV without being immunized.

The eight monkeys in the control group showed a severe loss of CD4 T-cells -- white blood cells typically targeted by HIV that play an important role in the immune system.

The vaccinated monkeys remained healthy for up to 14 months after their infection and displayed low or undetectable viral loads, without developing AIDS, said John Rose, a lead author of the Yale study.

Rose seeks permission from US health authorities to perform clinical trials of the "promising" vaccine, which can be administered without injection either orally or nasally, he said.

Researchers from the Yerkes Primate Research Center saw marked results in primates infected with cowpox modified with HIV genes. Of the 20 monkeys infected, 19 experienced a drop in HIV infection to undetectable levels, the Atlanta, Georgia-based researchers said.

Professor Dan Barouch and his colleagues at Harvard Medical School also presented promising results Thursday with a vaccine based on two AIDS virus genes coupled with the injection of a gene stimulating production of interleukine 2, which reinforces immuno-defenses.

Monkeys that were vaccinated and infected with HIV are still in good health two years later, they said.

"There is a new optimism in the international scientific community that an AIDS vaccine will ultimately be possible, although it will not be available soon," said Professor David Baltimore who leads the government research committee into finding a vaccine.

More than 360 specialist studies are to be presented in Philadelphia in the four-day conference to end Saturday, covering all aspects of the development of a vaccine, discoveries about the virus, and difficulties encountered by researchers in testing prototypes of an AIDS virus on people.

The AIDS virus has infected more than 58 million people since the start of the pandemic, causing the deaths of 22 million, according to the latest statistics from the United Nations.

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