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By Julie Steenhuysen Julie Steenhuysen – Tue Nov 3, 5:22 pm ET
CHICAGO (Reuters) – Swine flu can cause severe disease in people of all ages and appears to pose a special threat to those who are obese, according to an analysis of H1N1 cases in California released on Tuesday.

Public health researchers analyzed the state's first 1,088 hospitalized and fatal cases of H1N1 infection between April 23 and August 1.

Like other studies, they found the average patient who was hospitalized with H1N1 flu was younger than what is commonly seen with seasonal flu, but they also found severe disease at both ends of the age spectrum.

"What our study shows was that once you were hospitalized, if you were elderly you have a higher risk of dying," Dr. Janice Louie of the California Department of Public Health in Richmond, whose study appears in the Journal of the American Medical Association.

Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention, said the study matches the CDC's own observations -- that H1N1 affects all age groups, including people over 65.

"If they get it, it can be every bit as severe as seasonal flu, consistent with other data," Frieden told a news briefing.

"It does emphasize that providers should think of H1N1 influenza in all age groups," he said.

Frieden said the new findings do not change the CDC's recommendations for vaccination, which focus on younger people, those with underlying conditions such as asthma and pregnant women.

What it does suggest is that doctors need to be aware of the risks to their older patients if they do become infected, Louie said.

NOT A MILD DISEASE

"One of the perceptions we've been trying to dispel is that this is a mild disease," she said in a telephone interview.

"This can be very severe. In this paper, 30 percent of patients required intensive care."

Overall, 11 percent of people who were hospitalized died, but among people 50 and older, 18 to 20 percent died.

The most common causes of death were viral pneumonia and acute respiratory distress syndrome.

As with other studies, obesity appeared to play a significant role in the severity of disease.

In the 268 cases of adults over 20 whose weight was known, 58 percent were obese, with a body mass index of over 30, and of these, 67 percent were morbidly obese, with a BMI of over 40.

BMI is equal to weight in kilograms divided by height in meters squared. A person 5 feet 5 inches tall becomes obese at 180 pounds (82 kg).

"There definitely is something that is standing out as far as the obesity issue," Louie said.

"We certainly don't see the same thing with seasonal flu."

Louie said in California, the flu has caused shortages of antiviral drugs and of N95 respirator masks, but so far, based on her contact with doctors in the state, swine flu has not overwhelmed hospitals.

A report by Trust for America's Health suggested a mild pandemic could send as many as 168,025 people in California to the hospital.