The HIV vaccine, known as HVTN100, will be trialed in South Africa later this year, after preliminary testing showed promise.
The experimental vaccine came after a 2009 trial in Thailand called RV144. This was the first major success, after years of struggling to invent a vaccine.
One year after the 2009 tests, the effectiveness of RV144 was evaluated at 60 percent. However, after 3.5 years after the vaccination tests, the effectiveness reduced to some 31 percent, leading researchers to a new goal: raise the effectiveness of a vaccine back to a 60-percent minimum, and keep it there.