A new paper by Dr. Eyal Shahar says studies estimating flu vaccine effectiveness are biased in how they treat illness or clinical visits soon after vaccination. It cites a CDC analysis that estimated about 40% effectiveness in children and 22% to 34% in adults.
WHO:Dr. Eyal Shahar (University of Arizona); Dr. Shahar’s paper; CDC researchers (including a corresponding author referenced in the report)
WHAT:A paper argues flu vaccine effectiveness studies are biased by how they classify people who become ill or seek care within 13 days after vaccination.
WHEN:Reported Sep 1, 2026 at 5:30 PM ET
WHERE:California
WHY:The paper says the studies are biased because they handle the first 13 days after vaccination differently when counting people as unvaccinated or excluding those who develop symptoms or seek care.
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