Sept. 7, 2010 -- In adults and children over age 6 months, the H1N1 swine flu was no more severe -- and posed no greater risk of serious disease -- than recent seasonal flu bugs.
The findings come from Edward A. Belongia, MD, and colleagues at Wisconsin's Marshfield Clinic, who have been tracking flu cases since 2007.
Because nearly everyone in the surrounding community gets their health care through Marshfield Clinic -- and because Belongia’s team has been carefully testing everyone who comes in with flu symptoms -- the researchers have an extraordinarily detailed record of how flu behaves each year.
When the 2009 H1N1 swine flu pandemic swept through their community, Belongia and colleagues kept on collecting data. This allowed them to compare the 2009 H1N1 swine flu pandemic to the 2007-2008 and 2008-2009 flu seasons.
The researchers identified 545 people who had the 2009 H1N1 swine flu, 221 people who had a different seasonal H1N1 flu bug during the 2008-2009 season, and 632 people who had the H3N2 flu during the 2007-2008 season.
Belongia and colleagues find that H1N1 swine flu wasn't unusually bad:
1.5% of children with H1N1 swine flu were hospitalized, compared to 3.7% of children with seasonal H1N1 flu and to 3.1% of children with seasonal H3N2 flu.
4% of adults with H1N1 swine flu were hospitalized, compared to 2.3% of those with seasonal H1N1 flu and to 4.5% of adults with seasonal H3N2 flu.
2.5% of children with H1N1 swine flu had pneumonia, compared to 1.5% of children with seasonal H1N1 flu and to 2% of children with seasonal H3N2 flu.
4% of adults with H1N1 swine flu had pneumonia, compared to 2.3% of those with seasonal H1N1 flu and to 1.1% of adults with seasonal H3N2 flu.
And people who got the H1N1 swine flu didn't feel any worse than people who got seasonal flu. When patients ranked the severity of their flu symptoms, those who had the H1N1 swine flu reported less severe illness than those who had either of the recent seasonal flu bugs.
None of the Belongia findings apply to children under age 6 months. That's because their study originally was designed to measure the effectiveness of seasonal flu vaccine, and kids under 6 months are too young to get this vaccine.
"We found that children were disproportionately affected by 2009 H1N1 [swine flu] infection, but the perceived severity of symptoms and risk of serious outcomes ... were not increased in children with 2009 H1N1 relative to seasonal influenza A viruses," Belongia and colleagues conclude.
The findings appear in the Sept. 8 issue of TheJournal of the American Medical Association.