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A new phase of trials to test two promising HIV vaccines began last week in London and Oxford. The vaccines—one made of naked DNA, the other a weakened pox virus—contain genes from a common strain of HIV in Africa, and should provoke an immune response that fends off the virus itself. Volunteers are receiving different combinations of the vaccines or a placebo. Teams at Imperial College in London and the Medical Research Council’s Human Immunology Unit in Oxford hope to recruit 120 volunteers aged 18 to 60, who must be HIV-negative.