The near 40-year quest for an Aids vaccine received a hopeful boost on Saturday when scientists announced that a trial drug triggered an immune response in humans and shielded monkeys from infection.

Shown to be safe in humans, the candidate vaccine has now advanced to the next phase of the pre-approval trial process, and will be tested in 2,600 women in southern Africa to see whether it prevents HIV infection.

While the results so far have been encouraging, the research team and outside experts warn there are no guarantees it will actually work in the next trial phase dubbed HVTN705 or “Imbokodo” — the isiZulu word for “rock”. The results of the Imbokodo trial are expected in 2021-22.

“Although these data are promising, we need to remain cautious,” said study leader Dan Barouch, a Harvard Medical School professor. “This is only the fifth HIV vaccine concept that will be tested for efficacy in humans in the 35+ year history of the global HIV epidemic,” added Barouch.

Only one so far, RV144, yielded some protection. RV144 was reported in 2009 to reduce the risk of HIV infection among 16,000 Thai volunteers by 31.2 per cent — deemed insufficient for the drug to be pursued.

For the latest study, published in The Lancet medical journal, Barouch and a team tested the candidate drug on 393 healthy, HIV-free adults aged 18 to 50 in east Africa, South Africa, Thailand, and the United States.

The vaccine “induced robust (high levels of) immune responses in humans”, said Barouch.