NEW YORK (Reuters Health) - It's quite common for children who catch the flu to end up in the hospital, and it is not only those with other illnesses who are at risk, researchers report.

"Influenza used to be considered a disease of the old and infirm," lead researcher Dr. Susan E. Coffin told Reuters Health. "However, we found that both healthy and chronically ill children can become seriously ill with influenza."

Coffin, at Children's Hospital of Philadelphia, and colleagues studied data on 745 children who were hospitalized with confirmed influenza over a 4-year period. Overall, the researchers report in the medical journal Pediatrics, about 7 out of every 10,000 inner-city children were hospitalized for this reason annually.

Their average age was just under 2 years, but a quarter were less than 6 months old.

Nearly half had conditions such as asthma, heart disease, and neurologic or neuromuscular disease. These children were at considerably greater risk of complications from influenza infection, including pneumonia and respiratory failure, than were previously healthy children.

The researchers conclude that children with underlying conditions are among those who should receive high priority for flu vaccination.