Health officials released the surprising results at a news conference on Tuesday, noting that 46 per cent of 1,400 hospitalized adults did not have a chronic underlying condition.
They have said before that the majority of swine flu patients who develop severe illness have some sort of pre-existing condition, but the new data suggest the majority may be slimmer than was previously thought.
A study of 272 hospitalized swine flu patients, released by the New England Journal of Medicine earlier this month, concluded that 83 per cent of adults and 60 per cent of children had underlying conditions.
However, health officials cautioned that the new analysis is preliminary and did not count obesity as an underlying condition. Earlier research has suggested obesity could be a separate risk factor for severe swine flu illness. Further analysis that counts obesity could change the results, said a spokesman for the Centers for Disease Control and Prevention.
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The CDC looked at 1,400 adults and more than 500 children with swine flu who were hospitalized in 10 states at medical centers participating in a special disease surveillance network. The hospitalizations occurred from April through the end of August.
Looking at a larger number of hospitalizations was important because "we wanted to make sure that we weren't missing some important underlying conditions that we hadn't talked about earlier," said Dr. Anne Schuchat, who heads the CDC's National Center for Immunization and Respiratory Diseases.
Of the adult cases, about 26 per cent had asthma, 8 per cent had some other chronic lung disease, 10 per cent had diabetes, nearly 8 per cent had weakened immune systems, and 6 per cent were pregnant.
Not as much analysis has been completed on the children's cases, but health officials noted that 6 per cent were kids with sickle-cell disease or another condition from the same family of blood diseases.
The new virus, first identified in April, is a global epidemic. The CDC doesn't have an exact count of all swine flu deaths and hospitalizations, but existing reports suggest more than 600 have died and more than 9,000 have been hospitalized. Health officials believe millions of Americans have caught the virus.
The virus is hitting young people harder. Experts believe older people are suffering from it less, perhaps because they have a bit of immunity from exposure over the years to somewhat similar viruses.
On Tuesday, Schuchat said that five additional pediatric swine-flu deaths have been reported since late last week, bringing to 81 the total count of U.S. children who have died with the infection.
States have ordered almost 6 million doses of swine flu vaccine in the vaccination campaign that started last week. About half the available doses are shots and half are the nasal spray version of the vaccine, Schuchat said.