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.
The numbers again highlight how the young seem to be particularly at risk of catching the new virus. But data also show that the flu has been more dangerous to adults who catch it.
The average age of swine flu patients is 12, the average age for hospitalized patients is 20, and for people who died, it was 37. It seems to be deadliest to people 65 and older, with deaths in more than 2 percent of elderly people infected, Finelli said.
Also at the meeting, CDC officials made projections about flu vaccines expected to be available to protect against both seasonal and swine flu this fall.
More than 25 million doses of seasonal flu vaccine should be available by early September, CDC officials and vaccine manufacturers said.
The same five manufacturers that make the seasonal vaccine are producing swine flu vaccine as well. As many as 60 million doses of vaccine to protect against the new virus could be ready by September, said Robin Robinson, an official with the federal agency that oversees vaccine manufacture and distribution. That prediction seemed a bit optimistic, others at the meeting said.
The vaccinations might be given as two shots, spaced 21 days apart. But the vaccine has to be tested before it's made available to the public.