The road to an HIV vaccine hit another gully when the U.S. government canceled a trial of its most advanced experimental formula to date. The two-shot injection was similar to one that Merck developed and scrapped in 2007, after initial results showed that not only did it fail to protect against HIV, but in some cases it actually increased the risk of infection. Both vaccines were designed to test a new approach, one based on activating the body's cell-based immunity, in which killer immune cells take a more dominant role than antibodies in attacking HIV. Experts now believe not enough is known about this strategy to make a large-scale trial useful — yet. Researchers are hopeful the approach still holds promise; smaller studies are under way to work out how best to harness the body's virus-fighting cells.