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30%–41% effective against outpatient visits and about 30% against hospitalizations. By Flu Strain:
30%–42% effective depending on age and setting. Influenza B: 45%–71% effective. Why Effectiveness Varies How Effective is the Flu Vaccine?
38%–41% effective against outpatient visits and 41% against hospitalizations. 30%–41% effective against outpatient visits and about 30%
The primary reason for lower effectiveness this season is a with the dominant H3N2 "subclade K" strain. This mutated version emerged after the 2025–26 vaccine composition was already finalized. Other factors include: CDC Seasonal Flu Vaccine Effectiveness Studies How Effective is the Flu Vaccine?
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