A 26-year-old Philadelphia woman has died of complications from the so-called swine flu, city and state health officials said yesterday, the second such death in Pennsylvania.

The Philadelphia Department of Public Health declined to release additional information about the woman's death Sunday, except to say she was "very ill" when hospitalized.

There was no indication that the woman had any underlying condition that might have made her more susceptible to the flu, a state Health Department spokeswoman said.

City Health Commissioner Donald F. Schwarz said he could not comment on specifics of the woman's case, citing privacy rules. But he explained that this type of flu, while it has killed few people, is more likely to have grave consequences for younger people. Older people are likely to have some immunity to it because they were exposed to similar subtypes of this flu decades ago, he said.

On Thursday, a 55-year-old Berks County woman was the first Pennsylvanian to die of complications related to the illness, a novel type of H1N1 influenza. As of yesterday afternoon, the state Health Department had recorded 78 cases of this flu strain in Philadelphia, with nine probable cases still under investigation.

The 26-year-old Philadelphia woman is at least the 29th confirmed swine flu-related death nationwide. On Friday, Gov. Rendell said she was in a city hospital in critical condition, though he mistakenly referred to her as a man. The governor said the patient waited too long to go to the hospital. Schwarz said the warning signs for when a person should go to a hospital are the same for H1N1 as for any other illness, and may include chest pain and trouble breathing.

"The sad loss of this 26-year-old woman reminds us that influenza is a serious disease that can cause complications in any age group," Pennsylvania Secretary of Health Everette James said in a released statement.

The new flu has largely targeted young people and spared the very old - opposite the pattern of seasonal flu.

In Pennsylvania, for example, the Health Department said that more than 80 percent of the cases through Thursday had been in people younger than age 30, including 6 percent in children 4 or younger; 21 percent ages 5 to 9; 31 percent ages 10 to 14; 12 percent ages 15 to 19; and 13 percent ages 20 to 29.

In Radnor Township, more than 70 students at Ithan Elementary School called in sick yesterday, many with a fever, according to a letter from Superintendent Linda Grobman. There were no confirmed flu cases among those, a district spokeswoman said.

Contact staff writer Zoe Tillman at 215-854-2917 or ztillman@phillynews.com.