22%–34% effective against outpatient visits and 30% against hospitalizations.
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?
38%–41% effective against outpatient visits and 41% against hospitalizations. How Effective is the Flu Vaccine?