The first large-scale trial of an HIV vaccine is set to begin in South Africa, it has been announced.
South Africa has one of the most severe HIV epidemics in the world
Three thousand HIV negative men and women who are sexually active will be immunised in the four-year study.
An international team of researchers, led by experts from the US, will oversee the trial of the vaccine, created by the drug company Merck.
It is hoped the study will provide information about how a vaccine will work in a heterosexual population.
It should also show if it is effective among women.
The test vaccine has already been through trials for safety and immune response in the Americas, Africa and Australia.
It does not contain live HIV, so cannot cause infection, but does contain copies of three HIV genes. The hope is that exposure to these genes prompts an immune response in the body so that cells containing HIV virus would be recognised and destroyed.
The study, jointly run by the international HIV Vaccine Trials Network (HVTN) and the South African Aids Vaccine Initiative (SAAVI) is also designed to show if the vaccine, which is based on the B strain of HIV, has the potential to protect against the C strain of the virus, which is the subtype prevalent in South Africa.
All those who take part will be aged 18 to 35. No pregnant women will be involved in the trial.
Some will be given the vaccine, while others will be given a dummy version of the jab.
Everyone will receive advice about how to practise safe sex.
The trial has been approved by the South African Medicines Control Council and the South African Department of Agriculture, and has been reviewed by the US Food and Drug Administration.
Even if the trial provides positive findings, there would have to be further studies before the vaccine could be licensed.
Dr Lawrence Corey, the lead researcher for the HVTN who is based at Fred Hutchinson Cancer Research Center in Seattle, who is running the study, said: "This trial will answer several major scientific issues that face all of us in the field of HIV-vaccine development.
"It will determine the usefulness of vaccines that induce high immune response to the parts of the virus that are similar between different strains of HIV."
Dr Glenda Gray, of the Perinatal HIV Research Unit at the University of Witwatersrand in Johannesburg, who is also working on the study, said: "Our communities here in South Africa are faced with the burden of HIV on a daily basis, and the trial investigators and study team have spent years developing a rapport with the community so that together we can move forward in our quest to identify improved approaches to prevent new HIV infections."
Deborah Jack, chief executive of the UK's National Aids Trust said: "With levels of HIV rising globally, new ways of effectively preventing HIV are urgently needed.
"The development of a safe, effective and accessible vaccine could have an enormous impact on rates of HIV infection worldwide.
"Whilst vaccine development is a long and complex process, we welcome the announcement of this new trial as a further step towards the ultimate goal of stopping the spread of HIV."