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Courtesy: Reuters

A shot that helps keep AIDS-infected monkeys alive may offer the best clues yet about how to make , researchers reported.

The experiment provided important clues about how the AIDS virus destroys the immune system, and how to track the health of infected people, the researchers said on June 08, 06 in Washington.

''A vaccine of this type does not appear to prevent infection,'' Dr. Norman Letvin of Harvard Medical School in Boston, who helped lead the study, said in a telephone interview. What the vaccine may do, he said, is help infected people live longer without becoming ill.

The study, published 25 years after the first medical description of AIDS was released by the U.S. Centers for Disease Control and Prevention, highlights the biggest challenge of the pandemic -- how to prevent infection.

In the 1980s, medical researchers believed it would be easy to make a vaccine against the virus. But more than billion later, no one has an effective vaccine, although more than 30 different ones are in various stages of testing in people.

The problem is that the virus, which has killed more than 25 million people since 1981, attacks the immune system cells that usually defend against infection. It mutates constantly, making a moving target.

Letvin's team vaccinated monkeys against simian immunodeficiency virus or SIV, using monkeys and a monkey version of the AIDS virus that are considered the closest model of human infection.

Most vaccines stimulate the body to produce antibodies, which help orchestrate an immune response against a particular virus or bacteria. But that approach does not work for HIV.

Scientists believe a second type of immune response, called cell-mediated immune response, is necessary to fight the AIDS virus. Letvin's team tested a vaccine that triggers a strong immune response by those cells, known as T-cells.

TRIPLING LIFE SPAN Monkeys who got the SIV vaccine lived much longer when they were later infected with HIV, living up to 900 days, while unvaccinated monkeys died on average within 300 days, Letvin's team reports in Friday's issue of the journal Science.

''There are two human vaccines that are similar to this that are now going forward into advanced efficacy trials,'' Letvin said. One is being developed by the U.S. National Institutes of Health and the other by Merck and Co.

The trial gives researchers something to look for in their human volunteers -- who cannot be deliberately infected with the AIDS virus as the monkeys were.

''The magnitude of the immune response that is generated by vaccination predicts how long the animals will live after infection. The more potent the immune response after injection, the longer the monkey lived,'' Letvin said.

Also, how much virus is in the blood, called viral load, is not particularly important, Letvin said. That runs counter to many assumptions among AIDS experts.

''What is useful (to measure) is the subpopulation of helper CD4 T cells -- the central memory CD4 T lymphocyte population,'' Letvin said.

''This tells us something profoundly important about why AIDS progresses clinically -- the preservation of this central memory population of CD4 helper lymphocytes appears to be absolutely crucial for maintaining immunological competence,'' Letvin said.

Left untreated, the AIDS virus gradually destroys the immune system and patients die on average within nine to 10 years of various infections or cancer.

Drug cocktails can delay that but they are often expensive or unavailable to people in the most badly affected countries.

HIV infection is always fatal and there is no cure.