Scientists in the United States have tested a vaccine on monkeys that appears to be effective in preventing the simian equivalent of HIV.
Research published in the journal Nature showed the vaccine enabled half the monkeys tested to clear from their bodies an infection of the Simian Immunodeficiency Virus (SIV).
The vaccine was effective in nine of the 16 monkeys inoculated.
The scientists say at first the infection began to establish and spread, but then the monkeys' bodies started to respond by searching out and eradicating all signs of SIV.
The monkeys that responded successfully were still free of infection between one-and-a-half and three years later.
The team says they are trying to establish why the vaccine only worked in half the monkeys tested, but hope this could pave the way for human clinical trials in two years after stringent safety tests.