(USA TODAY) -- The first analysis of children who have died of the new H1N1 flu virus shows that two-thirds had severe existing medical conditions, nearly half also got bacterial infections, and a dozen were never treated with Tamiflu, the government reported Thursday.

At least 477 people, 40 of whom were children, have died of H1N1 influenza, also called swine flu, since it emerged in the United States in April. An analysis of 36 of the children's deaths found that 81% were age 5 or older, and 67% had high-risk medical conditions, including epilepsy, cerebral palsy, muscular dystrophy and other neuro-developmental disorders, the U.S. Centers for Disease Control and Prevention reported.

In a typical flu season, half or more of children who die are 4 years old or younger. About a third of children who die suffer from neurological disorders.

CDC Director Thomas Frieden said Thursday that the study highlights the importance of treating children with special needs and neurological disorders promptly with antiviral drugs if they develop flu symptoms.

It's also critical, he said, that "they're at the front of the line for flu vaccination when it becomes available." The first doses of flu vaccine are still expected to be ready for distribution by mid-October, Frieden said.

At least 12 of the children who died between April and August never received the best available flu treatment, oseltamivir, sold as Tamiflu, the study found. In five other cases, CDC officials couldn't determine whether the children got Tamiflu. Other children weren't given Tamiflu until long after they became ill. A 1-year-old African-American boy, for instance, didn't get Tamiflu for 23 days after his symptoms began, CDC reported.

Lena Napolitano, an intensive care specialist at the University of Michigan who has treated more than a dozen severely ill swine flu patients, says the analysis suggests that many physicians didn't suspect H1N1 virus caused the children's illness, adding that doctors shouldn't wait until test results come in before starting treatment.

Ten of 23 children for whom test results were available developed bacterial infections on top of their viral infections, a double blow that can lead to toxic shock and death, CDC reported.

Frieden noted that the pandemic, which simmered all summer, is heating up again, especially in the Southeast, where many schools began early. Flu has already forced school closures in two states. Twenty-three schools have closed in Tennessee and another in Indiana, affecting a total of 13,299 students, Justin Hamilton of the Department of Education said Thursday. Nineteen of the schools are in Blount County, Tenn., south of Knoxville.

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