South Africa has launched a major trial of a vaccine to protect against HIV, the virus that causes AIDS.  Some scientists believe the vaccine could be the most promising development since the HIV virus was identified in 1983.

"If we have a 50 percent efficacy rate, we would consider this an effective vaccine," said Glenda Gray, president of the South Africa's Medical Research Council.  "A vaccine is critical for South Africa," she added.  That would be a significant step up from used in a trial in Thailand in 2009, which maintained a protection rate of about 30 percent for three-and-a-half years after the first jab.  Results from the South African trial are expected in four years.

Scientists have already determined that the vaccine is safe after an 18 month trial on 252 patients.  This step is to determine if it is indeed a virus-killer.  About seven million people in South Africa are living with the virus, and officials aim to enroll 5,400 of them to take part in the study. 

The South African Medical Research Council is getting help in the new trial from the US National Institutes of Health, the Bill and Melinda Gates Foundation, Sanofi Pasteur, GlaxoSmithKline, and the HIV Vaccine Trials Network.