A successful attempt by a group of Harvard researchers at protecting monkeys against the AIDS-causing virus has introduced a new strategy by which scientists may create a human AIDS vaccine.

Using a weakened form of SIV, an AIDS-causing virus in monkeys, Medical School Professor of Microbiology and Molecular Genetics Ronald C. Desrosiers and his colleagues at the New England Regional Primate Center have successfully vaccinated several monkeys against the disease.

But while Muthiah D. Daniel, senior virologist at the New England Regional Primate Research Center and first author of an article which appeared last month in Science describing the study, called the vaccine "a novel technique," he also cautioned that it would be at least four or five years before a similar vaccine could be tested on humans.

Traditionally, doctors inject patients with live but mutated forms of viruses which cause diseases such as polio, measles and small pox in order to elicit a mild immune response. The body then relies on its immune "memory" to respond and effectively to another attack, and becomes protected against future infections.

But because mutations in HIV, the virus which causes AIDS in humans, readily mutate to their original form and regain potency, researchers have been unsuccessful in using this technique against the virus.

Instead of introducing a mutation, Desrosiers completely removed a gene from the virus. Mutations replacing missing genes are extremely rare, making a return to original potency less likely.

Four rhesus monkeys were injected with the altered SIV and remained healthy. After two years and three months, the monkeys resisted a low dose injection of live unaltered SIV.

Seven months later, two of the monkeys were challenged with a stronger dose, capable of killing 1000 monkeys, only to resist infection a second time.

Daniel said researchers are now removing multiple genes from both SIV and HIV to determine how many deletions a virus can sustain before becoming ineffective as vaccines. By removing as many genes as possible, researchers hope to further decrease the chances of reverse mutation.

After such work is compete, Daniel said a form of HIV lacking several critical genes will be introduced into chimpanzees, which are susceptible to the virus. If the chimpanzees resist these challenges, testing will begin on human volunteers.

But Desrosiers warned against overwhelming optimism in an accompanying Science news article, saying it will take at least 10 or 15 years of safety testing before the vaccine is used regularly in humans.