22%–34% effective against outpatient visits and 30% against hospitalizations.
30%–42% effective depending on age and setting. Influenza B: 45%–71% effective. Why Effectiveness Varies How Effective is the Flu Vaccine?
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?
30%–41% effective against outpatient visits and about 30% against hospitalizations. By Flu Strain: How Effective is the Flu Vaccine?