A vaccine against HIV is on the horizon after scientists showed a new drug triggered a protective immune response in humans and stopped two thirds of monkeys becoming infected.

In the 35 years since the HIV epidemic began, just four vaccines have been tested on humans, with the best only lowering infection rates by 31 per cent, leading to trials being discontinued.

But in what was described as "promising" and "an important milestone", an international team of scientists showed that the new vaccine boosted the immune systems of nearly 400 healthy adults.

And when vaccinated rhesus monkeys were exposed to the disease six times, only one third became infected.

Now scientists from institutions including Harvard, MIT and The National Institutes of Health have begun testing the vaccine on 2,600 women who are at risk of HIV in southern Africa and hope to have results by 2021.