The STEP Study was a Phase IIb clinical trial intended to study the efficacy of an experimental HIV vaccine based on a human adenovirus 5 (HAdV-5) vector. The study was conducted in North and South America, the Caribbean, and Australia. A related study (the "Phambili trial") using the same experimental vaccine was conducted simultaneously in South Africa. These trials were co-sponsored by Merck, the HIV Vaccine Trials Network (HVTN), and the National Institute of Allergy and Infectious Diseases (NIAID), and had an Oversight Committee consisting of representatives from these three organizations. In South Africa the trial was overseen by the South African AIDS Vaccine Initiative.[1]
These trials were terminated before their scheduled conclusion, when the Data Safety Monitoring Board determined that the vaccine was not preventing HIV infection, and was possibly enhancing susceptibility to HIV infection in some of the study participants.[2]