Scientists describe the current possibility of an HIV vaccine as “perhaps one of the most optimistic moments we have been in" since 1984. Three experimental HIV vaccine trials are set to enter the final testing stages. Dr. Susan Buchbinder, San Francisco Department of Public Health, via NBC News One of the trial vaccines, HVTN 702, is based on an earlier vaccine that showed a 30% effectiveness rate. A second trial vaccine, Imbokodo, is being tested on 2,600 women from five southern African nations. The third trial, Mosaico, will be tested in the U.S., Europe and Latin America on 3,800 gay men and trans people. Dr. Susan Buchbinder, San Francisco Department of Public Health, via NBC News Even if the trials fail, public health officials point to the effectiveness of prevention treatments, such as PrEP, to halt the spread of HIV.