Health departments in 45 states, New York City and the District of Columbia reported widespread influenza activity for the week ending Dec. 20, according to the latest data collected by the Centers for Disease Control and Prevention.
Pneumonia and influenza accounted for 7.8% of all deaths reported for the period, hitting what the CDC described as an "epidemic threshold" for the period.
These were the highest levels seen during the 2003-2004 flu season, which started about two months earlier than usual and has killed at least 42 children and an unknown number of adults.
Influenza, an infectious disease marked by respiratory inflammation, fever, muscular pain and intestinal tract irritation, kills an estimated 36,000 Americans and hospitalizes 114,000 every year.
About 92 children under the age of 5 die from flu-related illnesses in an average year, according to the CDC.
A CDC spokeswoman said there were indications in a few states that flu cases had peaked.
Flu season runs from October until April, with the peak usually occurring in the winter months.
Public anxiety this year is high because the virus hit the country hard in early October, before many Americans were vaccinated. A subsequent rush for flu shots was complicated by a shortage of vaccine in some parts.