It showed more than half of all hospitalizations were people 24 and younger; more than a quarter were ages 5 to 18 years.
"Essentially, this is still a young person's disease," said Dr. Anne Schuchat of the Centers for Disease Control and Prevention.
Swine flu deaths were concentrated in young and middle-aged adults. A third of all deaths were people ages 25 through 49; another third were 50 to 64.
Only 12 percent of deaths occurred in elderly. That's a stark contrast to the roughly 90 percent of deaths in the elderly from seasonal flu, Schuchat said at a Tuesday press conference.
"It's almost completely reversed," said Schuchat, who heads the CDC's National Center for Immunization and Respiratory Diseases.
The figures are similar to what the CDC saw in the spring, she said.
Many people 55 and older have some degree of immunity to the swine flu virus, perhaps from exposure decades ago to a similar virus or vaccine. But the ability of the swine flu virus to attack deep in the lungs seems to also make it more dangerous to some of the younger people who are infected, CDC officials say.
The latest figures show about 5,000 hospitalizations in 27 states for lab-confirmed swine flu, and about 300 deaths in 28 states. Not all states report lab-confirmed swine flu cases to the CDC; some report "influenza-like illnesses" that may include cases caused by other kinds of viruses.
The count did not include a breakdown of how many were pregnant or had other health problems that put them at higher risk for severe complications.
The CDC does not have an exact count of all the U.S. swine flu deaths and illnesses since the virus was first identified in April, but the agency says more than 800 have died, including at least 86 children. Millions of Americans have been infected, although many probably suffered only mild illness, CDC officials say.