Data from the CDC indicate that this year’s flu shot was about 48% effective, reports MedPage Today. 

The authors noted that most influenza infections this season have been caused by influenza A (H3N2). This virus poses “special challenges,” they said, because it undergoes more frequent and extensive genetic changes than either the H1N1 A or influenza B strains. Because of this, it requires more frequent vaccine updates to “maintain activity against evolving circulating strains.”

This year’s flu shot has been most effective against H3N2 A viruses among children ages 6 months to 8 years (vaccine effectiveness 53%, 95% CI 16%-74%) and adults 50-64 years old (50%, 95% CI 23%-67%).

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