Influenza killed an average 23,607 people each year in the past three decades, one-third fewer than the 36,000 estimate previously used to weigh the severity of a flu season, a U.S. study has found.

The 36,000 figure, cited by thousands of media stories during last year’s swine flu pandemic, was based on a 2003 report that examined data from the 1990s. That was a particularly deadly decade for flu strains and the resulting estimate overstated the effect of the flu, according to the study by the Centers for Disease Control and Prevention. Thursday’s research looked at data from 1976 to 2007.

Influenza is a rapidly evolving virus, and the severity of the season depends on which strains are circulating and how well a population has been inoculated. Annual deaths associated with flu ranged from 3,349 to 48,614 in the study. The new H1N1 strain that emerged last year killed about 13,000 people, though the effect on children and young adults was greater than a typical flu season.

The report “demonstrates the substantial variability in mortality estimates by year, influenza virus type/subtype, and age group,” researchers wrote in the CDC’s Morbidity and Mortality Weekly Report. “Future research that considers years-of-life lost is needed to better communicate the mortality burden of influenza in these younger populations.”