Nov. 3 (Bloomberg) -- Swine flu is almost three times more likely to kill patients older than 50 once they become hospitalized than those younger than 18, a study found.

Young people are still more likely than older adults to be infected, and the vast majority of H1N1 cases carry only mild symptoms. Among 1,088 people hospitalized in California, about 20 percent of those aged 50 and older died compared with 7 percent of those 18 and younger, according to a study today in the Journal of the American Medical Association.

The reason for the discrepancy among hospitalized patients in different age groups is that the older a person is the more likely they’ll also have other medical conditions, said William Schaffner, a liaison to the U.S. government’s vaccine advisory panel who didn’t work on the study.

“They’re much more likely to have a serious underlying illness such as diabetes, heart disease or underlying lung disease,” said Schaffner, chairman of the Department of Preventive Medicine at Vanderbilt University in Nashville, Tennessee, in an Oct. 30 telephone interview.

Those older than 50 may have some immunity to swine flu, also known as H1N1, which is why they aren’t getting sick as often as young adults and children, said study author, Janice Louie, of the California Department of Public Health, which conducted the research.

Healthy older adults should still wait to be vaccinated until more of the vaccine becomes available, Schaffner said. Children ages 6 months to 24 years and pregnant women are the highest priority groups for getting the swine flu vaccine, according to the Atlanta-based CDC’s guidelines.

530 People Killed

Swine flu, also known as H1N1 influenza, killed 530 people in 28 states from Aug. 30 to Oct. 24 and accounted for 12,466 hospitalizations nationwide, according to CDC.

In today’s study, researchers looked at state residents who were hospitalized or died with laboratory evidence of swine flu between April 23 and Aug. 11. Among 1,088 cases studied, 32 percent were in children younger than 18, with infants having the highest rate of hospitalization. Fever, cough and shortness of breath were the most common symptoms.

Overall, 118 people died, including eight children younger than 18 years and 51 people aged 50 and older. The most common causes of death were viral pneumonia and acute respiratory distress syndrome, the authors said.

The CDC said nationwide numbers aren’t available for how many people aged 50 and older have died of swine flu after being hospitalized with the virus. Last week the agency said that swine flu has killed 114 children in the U.S. since the outbreak surfaced in April, including 19 reported in the week from Oct. 18 to Oct. 24. Flu activity is now widespread in 48 U.S. states, the CDC said last week.

Underlying Conditions

Nearly 70 percent of those hospitalized in the study with swine flu had underlying conditions. In adults those conditions included asthma, diabetes and chronic obstructive pulmonary disease. In younger people the conditions included seizure disorders or cerebral palsy.

Of those hospitalized, 340 were admitted to intensive care units and a majority of the 297 with available information on them required mechanical ventilation.

Seasonal flu kills about 36,000 people a year in the U.S. People older than 64 years, younger than five or who have specific medical conditions have higher rates of hospitalization and death from seasonal flu, the authors said.

To contact the reporter on this story: Nicole Ostrow in New York at nostrow1@bloomberg.net.

Last Updated: November 3, 2009 15:57 EST