A report in the Centers for Disease Control and Prevention's Morbidity and Mortality Weekly Report also said despite the benefits of flu vaccination, only 40 percent of Americans age 6 months and older had reported getting a flu vaccine this season as of early November.
The estimated benefits of vaccination for the 2012-13 season were higher than any other season for which CDC has produced similar estimates. These high numbers were attributable to the severity of last year's flu season.
The report estimated that last season there were a total of 31.8 million influenza-associated illnesses, 14.4 medically attended illnesses and 381,000 hospitalizations in the United States.
"The estimated number of hospitalizations reinforces what we have always known about flu: that it is highly variable and can be very serious," Dr. Tom Frieden, director of the CDC, said in a statement.
"Most of estimated hospitalizations last season were in people 65 and older. This shows how hard a severe H3N2 season can hit this vulnerable group."
Children age 6 months through age 4 and persons age 65 and older are among those most vulnerable to influenza and accounted for an estimated 69 percent of prevented hospitalizations, the report said.
The report estimated if 70 percent of the U.S. population had been vaccinated last season, another 4.4 million flu illnesses, 1.8 million medically attended illnesses and 30,000 flu hospitalizations could have been prevented.