LANTA (AP) 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.

Regular seasonal flu sickens anywhere from 15 million to 60 million Americans each year.

The United States has roughly half the world's swine flu cases, with nearly 28,000 reported to the CDC so far. The U.S. count includes 3,065 hospitalizations and 127 deaths.

The percentage of cases hospitalized has been growing, but that may be due to closer scrutiny of very sick patients. It takes about three days from the time symptoms appear to hospitalization, Finelli said, and the average hospital stay has been three days.

Other health problems have been a factor in most cases: About one in three of the hospitalized cases had asthma, 16 percent diabetes, 12 percent have immune system problems and 11 percent chronic heart disease.