A study examining the effectiveness of a vaccine for the monkey equivalent of HIV, simian immunodeficiency virus (SIV), eradicated the disease in more than half the primates it was administered to.

Scientists from the Vaccine and Gene Therapy Institute at Oregon Health and Science University, who conducted the study, said they want to use a similar approach to test a vaccine for HIV in humans.

The study showed that nine out of 16 monkeys given the vaccine were able to clear SIV infection from their bodies.

The research team looked at an aggressive form of virus called SIVmac239, which is up to 100 times more deadly than HIV.

Infected monkeys usually die within two years, but in some inoculated primates the virus did not take hold.

Oregon Health and Science University’s Vaccine and Gene Therapy Institute Prof Louis Picker said: "It’s always tough to claim eradication, there could always be a cell which we didn’t analyse that has the virus in it. But for the most part, with very stringent criteria. There was no virus left in the body of these monkeys."

The vaccine tested was created based on another virus called cytomegalovirus (CMV), which is part of the herpes family.

Scientists used the highly infectious power of CMV to sweep through the body, but instead of causing disease, it was modified to rally the immune system into action to fight off the SIV molecules.

"It maintains an armed force, that patrols all the tissues of the body, all the time, indefinitely," said Prof Picker.

The rhesus macaque monkeys were administered the vaccine and then exposed to SIV.

At first the infection began to establish and spread; however, eventually the monkeys’ immune systems started to respond, searching out and destroying all signs of the virus.

"We have now engineered a CMV virus, which generates the same immune response but has been attenuated modified to lose its virulence to the point where we think it is unequivocally safe."

The monkeys that responded to the vaccine were still clear of infection between one-and-a-half and three years later. However, it is still a mystery why the vaccine only worked in around half of them.

Scientists are now testing to see if the vaccine can be used after SIV exposure to potentially cure infected monkeys.

They also want to establish whether the technique will work in humans but Prof Picker said they must make sure it is completely safe before they do so.

"We have now engineered a CMV virus, which generates the same immune response but has been attenuated [modified to lose its virulence] to the point where we think it is unequivocally safe," he said.

If this passes through regulatory authorities the scientists hope to start the first clinical trials in humans in the next two years.

The full study is published in Nature journal.

Image: The vaccine cleared the virus in nine out of 16 monkeys. Photo: courtesy of Wikipedia.