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

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, 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.

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