The increase is because of the aging of the population and a broader classification of flu-related diagnoses, CDC epidemiologist William Thompson says. Earlier estimates were based on the number of hospitalizations for pneumonia and influenza alone.

The new figures, based on data from the 1979-80 through 2000-01 flu seasons, reflect "the full effect of influenza," Thompson says, by counting hospitalizations for circulatory or respiratory illnesses in addition to influenza or pneumonia diagnoses.

Some cases might not be directly caused by flu but might include conditions that are worsened by the flu virus that is circulating, resulting in hospitalization.

The findings, reported in today´s Journal of the American Medical Association, emphasize the importance of receiving the flu vaccine, experts in infectious disease say. Last year, CDC researchers reported that flu causes an average of 36,000 deaths each year, a jump from the previous estimate of 20,000.

Flu complications are more severe in people over 50. The highest rates of hospitalizations are in people over 85, a population that doubled from 1976 to 2001, which explains much of the overall increase in hospitalization rates, the researchers report.

It´s not only the elderly who are at risk. Children under 5 are hospitalized at rates above those in the 50-64 age group. Last year, 152 children died of flu.

This year, for the first time, the CDC and American Academy of Pediatrics recommend that healthy children 6 months to 23 months old be vaccinated. They also urge shots for parents, siblings and others who have contact with babies from birth to 2 years old. The goal is to cut the flu´s impact and complications on the very young.

Flu not only can lead to pneumonia, says William Schaffner, a professor at Vanderbilt University and board member of the National Foundation for Infectious Diseases, but it also can worsen other medical conditions, such as diseases of the heart, kidney and liver.

"Studies have shown that when influenza vaccine is used widely, you not only get a reduction in hospitalizations due to pneumonia and influenza, but you also impact hospitalizations and deaths due to other illnesses," says Schaffner, who was not involved in the study.

The CDC judged the 2003-2004 flu season "moderately severe." One problem was that the vaccine used last year did not include the exact strain of flu that made most people sick.

This year´s vaccine is expected to match circulating strains, providing better protection.