An independent Data and Safety Monitoring Board (DSMB) met this week to review interim data from a large, international HIV vaccine clinical trial known as the STEP study — also referred to as the HVTN 502 or Merck V520-023 study. The clinical trial, which began enrolling volunteers in December 2004, is co-sponsored by the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health (NIH), and the pharmaceutical company Merck & Co. Inc., which also developed and supplied the candidate vaccine. Based on a review of interim data, the DSMB concluded that the vaccine cannot be shown in this trial to prevent HIV infection or affect the course of the disease in those who become infected with HIV (the vaccine itself cannot cause HIV infection because it contains only synthetically produced snippets of viral material). Therefore, Merck and NIAID instructed all study sites to cease administering the investigational vaccine but continue scheduled follow-up visits with all volunteers until the data can be more thoroughly evaluated and a course of action is developed. The same Merck candidate HIV vaccine is also being tested in South Africa by the HVTN and the South African AIDS Vaccine Initiative in a separate NIAID-sponsored clinical trial known as HVTN 503 or the “Phambili” study. This study was initiated in February 2007 and has enrolled 799 individuals. Immunizations and enrollment in the Phambili study have now been paused.