In what's being called a potentially "tectonic" shift in HIV treatment, a new clinical trial on an experimental vaccine that could prevent HIV will begin on Wednesday in South Africa.

The study, called HVTN 702, will be the largest HIV vaccine clinical trial ever to take place in South Africa.

5,400 sexually active men and women between the ages of 18 and 35 have volunteered to participate. At least two-thirds of the participants will be woman, who are more likely to contract the virus around these ages than men.

“If this study shows efficacy . . . this would be a tectonic, historic event for HIV,” Nelson L. Michael, director of the U.S. Military HIV Research Program, told The Washington Post.

Four years ago, researchers were encouraged to deepen their understanding of the virus when a test of a vaccine on 16,000 people in Thailand turned up a previously unknown vulnerability in the resilient pathogen.

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Unfortunately, the vaccine was only 31% effective and wore off over time, making it impossible to approve for the general public.

With more time, researchers plan to further develop their discovery and aim to ward off the virus that affects 1,000 people a day in South Africa.

“If deployed alongside our current armory of proven HIV prevention tools, a safe and effective vaccine could be the final nail in the coffin for HIV,” said Anthony Fauci, director of the United States’ National Institute of Allergy and Infectious Diseases, which has funded the trial. "Even a moderately effective vaccine would significantly decrease the burden of HIV disease over time in countries and populations with high rates of HIV infection, such as South Africa.”

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If the new vaccine proves to be 50% to 60% effective, experts say, that it would be sufficient for drugmakers to begin licensing negotiations with the South African government.

“HIV has taken a devastating toll in South Africa, but now we begin a scientific exploration that could hold great promise for our country,” Glenda Gray, president and chief executive officer of the South African Medical Research Council, said.

The aim of the new trial is to produce evidence for the regulators, obtain a licence for a vaccine, and to yield results sometime in 2020.