Atlanta - Swine flu has infected as many as 1 million Americans, U.S. health officials said Thursday, adding that 6 percent or more of some urban populations are infected. The estimate voiced by a government flu scientist Thursday was no surprise to the experts who have been closely watching the virus. "We knew diagnosed cases were just the tip of the iceberg," said Dr. William Schaffner, a Vanderbilt University infectious diseases expert who was in Atlanta for the meeting of a vaccine advisory panel. Lyn Finelli, a flu surveillance official with the Centers for Disease Control and Prevention, made the 1 million estimate in a presentation to the vaccine panel. The number is from mathematical modeling, based on surveys by health officials.