Sept. 4, 2009 -- The first detailed study of U.S. children killed by swine flu found the outbreak differs from ordinary flu in at least one puzzling respect: It appears to be taking a higher toll on school-age youngsters than on babies and toddlers.

At least 40 children have died, accounting for about one in 13 U.S. swine flu deaths, scientists with the Centers for Disease Control and Prevention said Thursday. Two-thirds of those already had high-risk health problems, confirming what officials have been saying about who is most vulnerable to swine flu.

It is not clear whether the new virus is more dangerous than ordinary seasonal flu for kids, though some health officials suspect it is. But the analysis shows some preliminary and important differences:

* Normally, half or more of the children who die of the flu are 4 and under. But more than 80 percent of the kids who died with swine flu were 5 through 17. Dr. Beth Bell, a CDC epidemiologist, said that may be because older children spend time at school and summer camp, exposed to more people than younger children kept at home.
* Almost two-thirds of the children who died with swine flu had epilepsy, cerebral palsy or other neurodevelopmental conditions. In a previous flu season, only a third of the children who died had those conditions.
* Other germs, working with swine flu in a one-two punch, were a big danger. A bacterial infection on top of the flu virus played a role in most of the deaths of otherwise healthy children.