The number of toddlers vaccinated against influenza in the United States has jumped during the current flu season, but fewer older Americans are lining up for the shots, federal officials said on Thursday.

The findings, based on a survey conducted by the Centers for Disease Control and Prevention, suggest the federal government has only been partly successful in steering supplies of flu vaccine to those at highest risk of complications from the illness.

Millions of Americans were advised last fall to forego a flu shot as the nation anticipated a shortage of vaccine. Seniors, infants, pregnant women and people with a chronic condition, such as diabetes, were among those urged to get the vaccine.

On Thursday, the CDC said 57.3 percent of children between the ages of 6 months and 23 months were vaccinated between September 2004 and December 2004. A 2002 survey found only 7.7 percent in the same group were immunized.

Flu shots were not included in the recommended childhood immunization schedule until 2004, which could explain part of the rise in the vaccination rate.

Top health officials expressed delight that more parents were immunizing children in the high-risk age group.

“That represents a public health success,” CDC Director Dr. Julie Gerberding told a hearing of the House Government Reform Committee in Washington. She said there have been four pediatric deaths due to flu this season, far fewer than at the same point in the 2003-2004 flu season.

The good news was tempered by data showing many older Americans foregoing a flu shot. Only 59 percent of Americans 65 years or older had been vaccinated through December, compared to 65.5 percent in 2003. About 90 percent of the estimated 36,000 U.S. flu deaths occur among seniors.

The CDC did not say whether the drop in the elderly vaccination rate might be linked to vaccine supply problems.

U.S. health officials have been scrambling to find extra supplies since October when Chiron Corp. said it could not deliver any vaccine due to a contamination problem at its plant in Great Britain.

The company was supposed to supply about half of the 100 million flu shots needed in the United States this year.