How A Mutation Made This Year’s Flu Season So Bad

Summary of How A Mutation Made This Year’s Flu Season So Bad

by Science Friday and WNYC Studios

12m•January 26, 2026
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Overview of How A Mutation Made This Year’s Flu Season So Bad

This Science Friday episode (host Flora Lichtman) features Dr. Jennifer Duchamp, a pediatric infectious disease specialist at Mount Sinai, explaining why this flu season has been unusually bad: a mutated H3N2 subclade (called “K”) has drifted away from the strains used to make this year’s vaccine, driving high case counts and pediatric hospitalizations while reducing vaccine effectiveness against infection (though not against severe outcomes).

Key points and main takeaways

  • Scale of this season: more than 19 million people sickened in the U.S.; weekly pediatric hospitalizations at the highest rate in over 15 years.
  • Primary cause: antigenic drift in an H3N2 subclade called K; these are small mutations to viral surface proteins that reduced the vaccine’s match.
  • Vaccine performance: reduced effectiveness at preventing infection with the K subclade, but still provides partial protection and helps prevent severe outcomes (pneumonia, hospitalization).
  • Circulating strains: roughly 90% of circulating viruses at the time of the interview were H3N2 subclade K.
  • Vaccine uptake: low—about 43% of eligible children vaccinated so far this season—contributing to case numbers.
  • Treatments: antiviral medications exist and can shorten illness and reduce complications if started promptly.
  • Broader issues: research toward a “universal” flu vaccine (including mRNA approaches) exists but progress has been slowed; recent CDC policy changes around universal childhood flu vaccine recommendations are controversial and likely worsened uptake.

What caused the mismatch: antigenic drift and the H3N2 “K” subclade

  • Flu vaccine composition is decided months in advance using global surveillance (Southern Hemisphere data often precedes Northern Hemisphere seasons).
  • The H3N2 strain developed mutations (antigenic drift) in key surface proteins after vaccine strain selection, producing the K subclade that the vaccine did not cover well.
  • This is not a brand-new strain like 2009 H1N1 (an antigenic shift) but a smaller, meaningful drift that reduced vaccine match.

Why H3N2 is especially problematic

  • H3N2 tends to mutate more readily and is historically the trickiest strain for vaccine matching.
  • When H3N2 predominates, vaccine effectiveness fluctuates more compared with other strains.
  • Even with drift, vaccines often retain enough similarity to protect against severe disease.

Policy, uptake, and public-health impact

  • The CDC’s change to stop universally recommending childhood flu vaccination (mentioned in the interview) has been criticized by pediatric infectious-disease clinicians; children are among the highest-risk groups for hospitalization.
  • Lower vaccine uptake since COVID-19 and annual fatigue (people skipping yearly shots) contributed to low coverage this season.
  • Reduced uptake plus a vaccine mismatch amplifies case numbers and strain on hospitals.

Clinical implications and practical recommendations

  • Get vaccinated: despite the mismatch, flu shots reduce severe outcomes and are still recommended.
  • Test early if symptomatic: testing enables timely antiviral therapy.
  • Start antivirals early: neuraminidase inhibitors and other antivirals can shorten duration and lower risk of complications if begun promptly.
  • Protect high-risk groups: children and the elderly remain priority groups for vaccination and early treatment.
  • Isolation and common-sense measures: stay home when sick, reduce household exposure.

Notable quotes and soundbites

  • “So the so‑called super flu is a little bit sensational... in terms of case numbers, absolutely.”
  • “This is basically just what we call antigenic drift.”
  • “The flu vaccine does still protect against major complications of the flu.”
  • On CDC policy change: “That’s an unmitigated disaster... grossly irresponsible.”

Action items (quick checklist)

  • If eligible: get the flu vaccine this season (even if imperfect).
  • If ill: get tested and seek antiviral treatment as soon as possible.
  • Parents and caregivers: prioritize vaccination for children and monitor for severe symptoms requiring medical care.
  • Clinicians: consider antivirals early for high-risk patients and encourage vaccination.

Guest: Dr. Jennifer Duchamp (pediatric infectious disease specialist, Mount Sinai). Host: Flora Lichtman. Produced by Kathleen Davis.