That's a little higher than with ordinary seasonal flu, several experts said.
What is striking and unusual is that children and teens accounted for nearly half of these cases, including many who were previously healthy.
"Contrary to the perception among many people that this influenza, novel H1N1, is mild, these data vividly demonstrate that influenza can make you very, very ill," said Dr. William Schaffner, a Vanderbilt University flu expert and spokesman for the Infectious Diseases Society of America.
"Clearly, the best way to protect yourself and your family is to get vaccine as soon as it becomes available," said Schaffner, who had no role in the study but has consulted for swine flu vaccine makers.
The study was done by researchers at the U.S. Centers for Disease Control and Prevention, working with hospitals and state and local health departments. Results were published online Thursday by the New England Journal of Medicine.
A second study released by the journal revealed that swine flu had a profound effect on intensive care units in Australia and New Zealand from June through August _ winter months and the normal flu season in the Southern Hemisphere.
"They had a 15-fold increase in ICU care. That's a 1,500 percent increase," said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. "We're still heading into our flu season. Will we see this same big increase in cases over the next several months or will we have a peak in cases in October or November?"
In the U.S. study, researchers led by the CDC's Dr. Seema Jain identified 272 patients hospitalized for at least a day from April through mid-June, when the novel virus caused its first wave of cases. That's about one-fourth of the total hospitalizations for swine flu reported during that time, but researchers only studied lab-confirmed cases and patients who agreed to be part of the study.
Three-fourths of these patients had other health problems, such as diabetes _ typical of seasonal flu, too. However, only 5 percent were 65 and older; ordinary seasonal flu usually hits hardest in the elderly.
Seven percent were pregnant, even though pregnant women make up only 1 percent of the general population. Among hospitalized patients whose weight was known, 26 percent were very obese, confirming a risk factor suggested by other information from the outbreak.
Symptoms were different _ 39 percent had diarrhea or vomiting versus only 5 percent with regular flu. About 40 percent had pneumonia. Seven percent died, and all had been put on breathing machines.
About three-fourths of hospitalized patients were given Tamiflu or other antiviral medicines, although most did not get these within the two days of first symptoms, as doctors recommend. Survival appeared to be better among those who got treatment quickly.
"The use of antivirals is critical," Jain said. "Start them, start them early. The patients who are hospitalized should get them quickly."
Other experts caution against making too much of specific numbers from these early results.
"We don't know how good these numbers are. They've done a good job; it's the best that we've got," Osterholm said. "But there are deaths out there that are not being recognized as influenza _ only an autopsy would pick them up. And there are likely hospitalizations for flu that were missed as flu."
The Southern Hemisphere study involved 722 patients with confirmed cases of swine flu who were treated in intensive care units. More than 14 percent, or one in seven, died. That study also found a disproportionately large number of cases in pregnant women and the very obse.
On the Net:
New England Journal: http://www.nejm.org
Swine flu information: http://www.flu.gov