The nearly 40-year search for an HIV vaccine got a hopeful boost on Saturday when scientists announced that a trial drug triggered an immune response in humans and protected monkeys from infection.

Considered safe in humans, the vaccine candidate has now entered the next phase of pre-approval testing and is being tested on 2,600 women in southern Africa to see if it prevents HIV infection.

While the results so far have been encouraging, the research team and external experts warn that there is no guarantee that it will work in the next trial, called HVTN705 or "Imbokodo" – the isiZulu word for "rock".

"Although these data are promising, we need to remain cautious," study director Dan Barouch, a professor at Harvard Medical School, told AFP.

Just because it protects two-thirds of the monkeys in a laboratory trial does not mean that the drug protects humans. "Therefore, we must wait for the results of the … study before we know if this vaccine protects people from it or not HIV infection," he said.

The results of the Imbokodo study are expected in 2021/22.

"This is just the fifth HIV vaccine concept to be tested for human efficacy in the 35-year history of the global HIV epidemic," added Barouch.

Only one so far, RV144, provided some protection. RV144 was reported in 2009 to reduce the risk of HIV infection by 31.2 percent among 16,000 Thai volunteers – which is considered insufficient to prosecute the drug.

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

Participants randomly received one of seven vaccine combinations or a placebo "dummy" alternative. They received four shots for 48 weeks each.

Requires "bad"
The study used so-called "mosaic" vaccine combinations.

These combine parts of different HIV virus types to trigger an immune response against virus strains from different regions of the world – as the body infects invaders.

The vaccine "induced robust (high) immune responses in humans," Barouch said.

The tests also showed that the vaccine was safe. Five participants reported side effects such as stomach pain and diarrhea, dizziness or back pain.

In a separate study, the same vaccine provided complete protection against infection in two-thirds of 72 experimental monkeys, each receiving six injections of an HIV-like virus.

"I can not stress how urgent we need a vaccine … to get rid of HIV in the next generation," said Francois Venter of the University of Witwatersrand in South Africa.

To comment on the study, in which he was not involved, Venter appealed to caution.

"We've been here before, with promising vaccine candidates that have not been exhausted," he told AFP.

"This novel is new in many ways, so it's exciting, but we still have a long way to go."

Could be "phenomenal"
Jean-Daniel Lelievre from the Vaccine Research Institute in France said the vaccine was probably not the "definitive" version but could be "phenomenal progress".

The World Health Organization estimates that an estimated 37 million people are living with HIV / AIDS.

There are about 1.8 million new infections and one million deaths each year.

Almost 80 million people have been estimated to have been infected since the first diagnosis of the virus in the early 1980s.

About 35 million have died.

A vaccine has proved elusive because the human immunodeficiency virus (HIV) can easily mutate and hide in cells, thereby evading the immune system and reappearing only years later.

Currently, HIV-infected people depend on lifelong virus-suppressive antiretroviral treatment (ART) to stay healthy.

Condoms are still at the forefront of efforts to prevent infection – mainly through sex and blood contact – although more and more people use ART as a prophylaxis.

The latest results will follow the International AIDS Conference in Amsterdam from July 23 to 27.

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