One-third of the 153 American children killed by the flu during the 2003-04 season were dead within three days of getting sick, and many of the youngsters were perfectly healthy before they were stricken, government researchers reported.

Five percent of the victims died within a day, 31 percent died before getting medical care, and 10 percent died in the emergency room, the Centers for Disease Control and Prevention said in its first detailed report on flu deaths among children.

Nearly two-thirds of the deaths were in children under 5, and 12 percent were younger than 6 months.

Overall, the 2003-04 flu season was not particularly severe, but it got off to an alarmingly early start.

The lead researcher, epidemiologist Niranjan Bhat of CDC’s influenza branch, said two findings surprised him: how fast the illnesses developed, and the number of children who were perfectly healthy.

Only one-third had asthma or other respiratory conditions that would have put them at high risk for flu complications.

However, the CDC also found that one-fifth of those killed had other conditions that could have made them more vulnerable but were not previously linked to flu complications. Those included neurological and neuromuscular disorders such as cerebral palsy that can limit someone’s ability to cough up respiratory secretions.

Bhat said the total number of deaths among children was probably more than 153; the CDC did not ask states to report flu deaths until well into the season.

The study was reported in today’s New England Journal of Medicine.

Bhat said the findings led CDC’s immunization advisory committee, starting with last year’s flu season, to urge that all children 6 to 23 months old get flu shots.

Last year was the first time the CDC has tried to separate children’s deaths from the flu’s overall toll. Complications from the illness kill an estimated 36,000 Americans a year.