A vaccine against AIDS was a step closer Thursday after U.S. scientists showed that the body's natural defenses could be used to attack and control the virus.

Scientists from the New York-based charity International Aids Vaccine Initiative (IAVI) said the results of a 10-year project could lead to a single injection vaccine that would leave the immune system permanently on guard against the disease.

More than half of the monkeys injected with the monkey version of HIV responded to the vaccine, and scientists were predicting that its effectiveness would be tested on humans in clinical trials within five years.

"What's exciting about these findings is that for the first time a vaccine candidate has been able to fully control the virus in some animals," said Wayne Koff, chief scientific officer at the IAVI. "And surprisingly, the data suggested the possibility that the immune system could eventually eliminate the virus altogether."

The new approach does not prevent people from catching HIV, unlike conventional vaccines, but it trains the body's natural immune defenses to be constantly on alert for the virus and prevent it spreading after infection.

"Before this publication scientists had pretty much given up on the idea of a vaccine that could control HIV replication," said Professor Robin Shattock, an immunologist at Imperial College London. "This publication puts it firmly back on the agenda."