A simple oral vaccine has managed to completely block the monkey equivalent of HIV, and human trials are on the way.

Researchers say that the vaccine’s progress has been “surprising” and “unexpected”, and it has managed to block rectal infection with SIV (monkey HIV) in rhesus macaques, while also producing rapid re-suppression of viral load in monkeys who were previously infected with SIV.

But the designers of the treatment are still working out how it actually works.

The vaccine appears to function by stimulating the production of a previously unknown group of CD8 T-cells that stopped the monkeys’ CD4 cells from recognising SIV as a foreign invader, thereby preventing an immune response to SIV.

This suppressant effect - which works in the opposite way to a traditional vaccine - means that the SIV is deprived of the SIV-specific immune-activated CD4 cells it needs in order to proliferate and establish an infection in the body.

The vaccine consisted of inactivated virus cells administered alongside doses of familiar bacteria - in the first case the TB-suppressant bacterium BCG - and then with gut bacteria commonly used in probiotic supplements.

This suggests that if human studies replicate the success seen in monkeys (not a sure thing in vaccine studies) the vaccine could be administered in the form of a drink.

Two initial safety trials are now planned in humans.

In one, HIV-negative volunteers at low risk of HIV will be given the vaccine to see if it stimulates the same immune- and virus-suppressant responses.

In the other, HIV-positive volunteers on fully-suppressive antiviral therapy (ART) will be given the vaccine and then taken off ART six months later if test tube results suggest the vaccine has produced such responses.

The study is published here.