A bird flu that has killed six people in Hong Kong has not mutated into a more dangerous bug that could spread easily from person to person and cause a worldwide epidemic, new laboratory tests indicate. Federal experts say there is still need for caution, and scientists at the Centers for Disease Control and Prevention are working on a new vaccine. "It is good news for now," Dr. Kanta Subbarao of the CDC flu laboratory in Atlanta said. In Hong Kong, a 25-year-old woman died, the sixth person felled there by the bird flu since the outbreak began in May. At least 12 other people have been infected.