Obesity appears to be a risk factor on a par with pregnancy for developing complications from an infection with pandemic H1N1 influenza, according to the most comprehensive look yet at swine flu hospitalizations.

About a quarter of hospitalizations for such complications have been in people who were morbidly obese, even though such people make up less than 5 percent of the population. That five-fold increase in risk is nearly the same as the six-fold increase observed in pregnant women, according to the report in the Journal of the American Medical Association.

A team from the California Department of Public Health analyzed data from the 1,088 hospitalizations that occurred in the state from the outbreak of the pandemic this spring through Aug. 11 and found that the highest rate of hospitalizations occurred among infants, while the highest rate of deaths occurred among those over age 50. Overall, 118 of the hospitalized patients, 11 percent, died, and a fifth of those deaths occurred among patients over the age of 50.

Half of those who were hospitalized were obese and a quarter were morbidly obese, with a body-mass index or BMI over 40. For a person 5 foot, 7 inches tall, that correlates to a weight more than 260 pounds.

Overall, the results are similar to those seen by other U.S. health authorities and those abroad, said Dr. Thomas R. Frieden, director of the Centers for Disease Control and Prevention.

Researchers have seen anecdotal reports that the obese might be at greater risk of complications from infection, but it has never been clear whether this was a result of the obesity itself or of other risk factors associated with obesity, such as diabetes or heart disease. Most overweight people suffer from such problems, which are known risk factors for complications from seasonal flu.

And, in fact, two-thirds of those hospitalized with complications from swine flu had such underlying risk factors -- but that means a third of them didn't, said Dr. Janice K. Louie, the lead author of the new study.

It's not clear why.

The obese may have an increased susceptibility to infections or a reduced respiratory capacity, both of which would increase their risk, the team concluded.

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