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Between 2004 and 2007, the National Institutes of Health conducted a STEP study. The STEP Study is the name of a clinical trial to test an experimental human immunodeficiency virus (HIV) vaccine that aimed to stimulate production of immune system T cells that can kill HIV-infected cells. The study enrolled 3,000 participants at sites in Australia, Brazil, Canada, the Dominican Republic, Haiti, Jamaica, Peru, Puerto Rico, and the United States. On the basis of its first evaluation of vaccine efficacy, the findings were as follows: There were 24 cases of HIV infection among the 741 volunteers who received at least one dose of the investigational vaccine. There were 21 cases of HIV infection among the 762 volunteers who were vaccinated with a placebo. In volunteers who received at least two vaccinations: There were 19 cases of HIV infection among the 672 volunteers who received the investigational vaccine.