Gilead Sciences Inc. may learn this year whether its drugs for treating HIV can also stop people from catching the virus in the first place. The approach may help curb the AIDS pandemic in poor countries and bring Gilead $1 billion a year in additional U.S. sales, said Josh Schimmer, an analyst at Leerink Swann & Co. in Boston. Most investors aren’t alert to the potential benefit, he said. Researchers are compiling the first data from 10 trials involving more than 20,000 people, and initial results may be available in July.