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
22%–34% effective against outpatient visits and 30% against hospitalizations. How Effective is the Flu Vaccine?
30%–41% effective against outpatient visits and about 30% against hospitalizations. By Flu Strain: The primary reason for lower effectiveness this season
30%–42% effective depending on age and setting. Influenza B: 45%–71% effective. Why Effectiveness Varies 30%–41% effective against outpatient visits and about 30%
For the current , the vaccine is estimated to be 36% effective overall at preventing medically attended flu illness. While this is lower than the typical 40–60% range, it still significantly reduces the risk of severe complications, hospitalizations, and deaths. Key Effectiveness Data (2025–26 Season) Overall Effectiveness: 36% (interim estimate). By Age Group:
38%–41% effective against outpatient visits and 41% against hospitalizations.