Vaccine Effectiveness

The 2025-2026 flu season is concluding with one of the lowest vaccine effectiveness rates in more than a decade, with the CDC reporting effectiveness of approximately 25-30% in preventing adults from needing medical care, according to data released Friday. A viral mismatch between the vaccine strain and dominant H3N2 variant that emerged early winter caused the poor performance, with vaccination proving more protective in children at around 40% effectiveness against medical visits. Despite low effectiveness, the CDC estimates 27 million illnesses, 350,000 hospitalizations, and 22,000 deaths from flu this season—substantially lower than the prior year's 40 million illnesses and 520,000 hospitalizations at the same point.

CDC interim analysis from September 2025 to February 2026 shows this season's flu vaccine effectiveness at 22-34% against outpatient visits for adults and 30-41% for those 65+, with hospitalization protection at 30-31%, due to H3N2 strain mismatch. The season saw 27 million illnesses, 350,000 hospitalizations, and 22,000 deaths, lower than last year's peaks but with 101 child deaths, 85% unvaccinated. Despite flaws, experts like Dr. William Schaffner stress shots prevent severe outcomes, with adult rates at 46.5%. Vaccination campaigns will target improved strains for next season.

CDC studies from September 2025 to February 2026 estimate flu vaccine effectiveness (VE) at 22-34% against influenza A/B outpatient visits for adults and 30-41% for those 65+, with 30-31% against hospitalizations. Patrick Maloney, PhD, from CDC's National Center for Immunization and Respiratory Diseases, led the analysis. Low VE underscores need for improved formulations amid slumping protection. Findings inform next season's vaccine development.