At an ill-fated press conference in 1984, U.S. Health and Human Services Secretary Margaret Heckler boldly predicted an effective AIDS vaccine would be available within just two years.
But a string of failed attempts - punctuated by a 2007 trial in which a Merck vaccine appeared to make people more vulnerable to infection, not less - cast a shadow over AIDS vaccine research that has taken years to dispel.
A 2009 clinical trial in Thailand was the first to show it was possible to prevent HIV infection in humans. Since then, discoveries have pointed to even more powerful vaccines using HIV-fighting antibodies. Now scientists believe a licensed vaccine is within reach.