Flu Kills 23,607 in U.S. Yearly, One-Third Fewer than Previously Estimated

Influenza killed an average 23,607 people each year in the last three decades, one-third fewer than the 36,000 estimate previously used to weigh the severity of a flu season, a U.S. study 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 impact of the flu, according to the study by the U.S. Centers for Disease Control and Prevention. Today’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 impact 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.”

Swine flu was the first pandemic in 40 years, infecting one in five Americans and sweeping the globe at an unprecedented rate. Because the virus strain targets young people, comparisons of deaths to seasonal flu obscure the full impact of the pandemic, according to the CDC.

Killer H3N2

The season flu vaccine each year protects against the three strains that scientists believe are most likely to circulate. Each year, world health officials choose one variety from each of three families of flu strains: H1N1, H3N2 and type B. The worst tolls occur in years when H3N2 takes hold, when deaths are almost three times higher, according to today’s report. In the 1990s, H3N2 strains were prominent in eight of nine seasons.

About 90 percent of people who die in a typical flu season are ages 65 and older. Last year, the age mortality trend was reversed. Scientists speculate that older people had some immunity to swine flu from exposure to a similar virus when they were young, whereas children and young adults had little natural protection.

Last year two shots were need for flu protection: one against seasonal flu strains and a separate vaccination against the new flu. This year, the seasonal flu shot will return to normal, including protection against the swine flu strain as the typical H1N1 component, according to the CDC. Flu season in the U.S. lasts from November to March.

To contact the reporter on this story: Tom Randall in New York at trandall6@bloomberg.net.

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