What is causing the early flu season in the US?

The early flu season is driven by increased holiday travel, gatherings, and the spread of a new influenza A(H3N2) variant, subclade K.

How severe is the flu outbreak in New York?

New York City is reporting very high flu activity, with flu-related hospitalizations doubling each week.

Are this year's flu vaccines effective against subclade K?

While subclade K was not directly included in this season's vaccines, related strains offer some protection, reducing severe outcomes by 30-75%.

Home / Health / New Flu Variant Fuels Early, Aggressive Flu Season

New Flu Variant Fuels Early, Aggressive Flu Season

16 Dec, 2025

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Summary

Flu activity surpasses epidemic threshold nationwide due to new H3N2 variant.
New York City reports very high flu activity, hospitalizations doubling weekly.
Vaccination offers 30-75% protection, but new variant wasn't in initial vaccines.

New Flu Variant Fuels Early, Aggressive Flu Season

The United States is experiencing an unusually early and aggressive flu season, marked by a significant increase in activity driven by a new variant of influenza A(H3N2), subclade K. Data from the CDC reveals that outpatient visits for flu-like symptoms have surpassed the epidemic threshold, confirming the season's official onset. This surge coincides with increased holiday travel and gatherings, facilitating rapid virus transmission.

Several regions, notably the Northeast, are reporting moderate to very high levels of flu activity. New York City is particularly affected, experiencing very high levels and a doubling of flu-related hospitalizations weekly. Public health officials are closely monitoring subclade K, a strain previously observed to cause busy flu seasons in other parts of the world. This variant was not directly included in this season's vaccines, though related strains offer some protection.