Why was the flu vaccine less effective this season?

The flu vaccine was less effective this season because a new strain of H3N2, subclade K, which spread easily, was not well-matched to the vaccine's formulation.

What were the estimated flu cases and deaths this season?

This season, there were an estimated 27 million illnesses, 350,000 hospitalizations, and 22,000 deaths from the flu.

What flu strains will next winter's vaccine target?

Next winter's vaccine for the 2026-27 northern hemisphere flu season is recommended to be built to handle the subclade K strain.

Home / Health / Flu Vaccine Fails: New Strain Defeats Shot

Flu Vaccine Fails: New Strain Defeats Shot

13 Mar

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Summary

This season's flu vaccine was only 25-30% effective for adults.
A new H3N2 flu strain was not well matched to the vaccine.
27 million illnesses, 350,000 hospitalizations, and 22,000 deaths occurred.

Flu Vaccine Fails: New Strain Defeats Shot

The recent U.S. flu season concluded with significantly low vaccine effectiveness, marking one of the worst rates in over a decade. Health officials reported that the vaccine was only about 25% to 30% effective in preventing adults from seeking medical attention. For children, the effectiveness was around 40% in preventing doctor's visits or hospitalizations.

A primary factor contributing to this diminished protection was the emergence of a new flu strain, a variant of H3N2 identified as subclade K. This strain proved difficult for the vaccine to target effectively, leading to an intense surge in flu cases during the early winter months. New York City health officials described it as the most severe season in two decades.