540 children have died of swine flu, CDC estimates

The federal figures are especially striking because seasonal viruses tend to do most of their damage in the elderly. In contrast, swine flu is resulting in more deaths among the young, with the new national estimates concluding that 100 more children have died from the disease than adults 65 and older.

"I do believe the pediatric death toll from this pandemic will be extensive and more than we see from the seasonal flu," Dr. Anne Schuchat, a top flu specialist at the CDC, said during a telephone briefing with reporters from the agency's Atlanta headquarters.

Still, specialists in pediatrics and infectious disease sought to reassure anxious parents who have flocked to doctors' offices and public clinics in an often futile quest for flu shots and spray.

In most children, they said, the H1N1 virus doesn't appear to cause symptoms significantly worse than those produced by run-of-the-mill flu.

"You have to have a lot of respect for influenza, but this particular influenza is no worse than the regular one," said Dr. John Schreiber, a pediatric infectious disease specialist at the Floating Hospital for Children at Tufts Medical Center. "It's that so many people are not immune and are getting sick this year."

Laboratory tests show that as of last week, 144 children nationwide had succumbed to swine flu since April, a figure that eclipses the toll for any year since careful tracking of pediatric deaths began in 2004. In Massachusetts, only one of the 15 swine flu deaths has been recorded in a patient younger than 18.

Disease specialists have long suspected that the official death tally captures only a fraction of the virus' genuine impact, however. Flu testing can be imprecise, and a flu infection can unleash a cascade of heart and lung complications, making it difficult to determine the cause of death.

"In those cases, the immediate cause of death might be called respiratory failure, but what set up that chain of events was the influenza infection," said Dr. Lauren Smith, medical director of the Massachusetts Department of Public Health.

So researchers at the CDC developed a new method to better gauge the damage wrought by H1N1, relying on extrapolations based on meticulously gathered data from 62 counties in 10 states. Independent specialists were still poring over the new formula tonight. The CDC has come under fire for some of its earlier attempts to reflect the true trajectory and impact of influenza.

The new estimates project that a total of 22 million Americans have been infected with the H1N1 virus, causing 98,000 to be hospitalized and 3,900 to die.

Since the dawn of swine flu in Massachusetts, 63 to 65 percent of laboratory confirmed cases have been in children under 18, while the elderly have accounted for only 1 percent.

In past years at Children's Hospital Boston, it was rare to see a mid-autumn case of flu. "It's not a November disease at all in Boston," said Dr. Anne Stack, clinical chief in the hospital's emergency department. "But we are definitely seeing influenza here now, and as a result of that we're seeing an increase in our volume in the emergency department."

At the Massachusetts General Hospital for Children on Wednesday night, seven children suspected of having swine flu lay in the intensive care unit. The sickest children, their lungs invaded by the disease and its companion complications, sometimes have to be put on breathing machines.

While some of the children had existing conditions such as asthma that primed them for problems, others had been perfectly fine until they caught the flu, said medical director Dr. Peter Greenspan.

"And the problem is," he said, "the kids who it hits hard, it hits really hard, and they can go downhill pretty quickly."