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Scientists have developed a diluted version of their original vaccine with a protective injection, previously dubbed the “holy grail” of AIDS research.

Since the epidemic began almost four decades ago, only four have been tested on patients – with none proving a success.

The latest injection uses a form of the common herpes virus virus cytomegalovirus (CMV) that causes cold sores.

The virus was weakened so it did not cause serious disease and proceeded to wipe out simian immunodeficiency virus (SIV) – the monkey version of HIV – in 16 out of 27 rhesus macaques.

(Image: GETTY)

Nine out of 12 monkeys given a second dose of SIV could also still fight it off three year later.

Co-corresponding author Professor Klaus Fruh said: "This research, using rhesus CMV, provides potentially important insights into the design of a human CMV-based HIV vaccine.

"We significantly attenuated CMV and still got the same type of immune responses as with the wild version of this vaccine."

A less powerful vaccine will make it potentially viable for humans.

(Image: GETTY)

People are often infected with CMV without any trouble but it can wreak havoc on those with weakened immune systems – such as organ transplant patients.

It's also dangerous for pregnant women as it can cause congenital defects such as cerebral palsy, deafness and developmental delay.

Prof Louis Picker, corresponding author on both studies, said: "These papers are important because they recapitulate the previously reported unique CMV vector efficacy with a genetically modified vector that is highly attenuated and therefore potentially safer for clinical use.

"In addition, this new work demonstrates most vaccinated rhesus macaques that are protected against SIV can also be protected against a second challenge years after initial vaccination.

"This is a level of durability that would be very important for a human HIV vaccine."

The CMV vaccine has been licensed by San Francisco based Vir Biotechnology – and has been backed by the Bill & Melinda Gates Foundation.

It is already planning to lead a clinical trial with a human version of the therapy.

The report was published in two papers in the journal Science Translational Medicine.