Lyn Finelli, a flu surveillance official with the Centers for Disease Control and Prevention, voiced the estimate at a vaccine advisory meeting Thursday in Atlanta.
The estimate is based on mathematical modeling. Nearly 28,000 U.S. cases have been reported to the CDC, accounting for roughly half the world's cases. The U.S. count includes 3,065 hospitalizations and 127 deaths.
An estimated 15 million to 60 million Americans catch seasonal flu each year.
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 onset of symptoms 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.
The World Health Organization declared swine flu a pandemic earlier this month. As of last week, it said that more than 39,000 cases had been reported worldwide, with 167 deaths.