Flu Shots Found Effective Against the Subclade K Variant

With nine reported pediatric fatalities related to influenza this season, many are questioning whether the updated vaccines provide adequate disease protection.

A new preprint study from researchers at the University of Pennsylvania (UPenn) offers encouraging insights into the ongoing 2025-2026 influenza season, which has been dominated by a drifted variant of the H3N2 virus known as subclade K.

The study, posted on medRxiv on January 6, 2026, examined blood samples from vaccinated individuals to assess how well the current seasonal flu vaccine triggers antibodies against this emerging strain.

Subclade K, characterized by several key substitutions in the hemagglutinin (HA) protein—including mutations like K2N, S144N, N158D, and others, which emerged late in 2025- was not included in the Northern Hemisphere vaccine formulation.

These UPenn researchers found that while subclade K viruses are antigenically advanced compared to the vaccine strain, the 2025-2026 vaccine still elicited robust antibody responses in many participants.

Before vaccination, only about 11% of individuals had neutralizing antibodies against subclade K.

After receiving the flu shot, this figure rose to around 39%.

Lead researcher Scott Hensley, a professor of microbiology at the UPenn, described the results as "unexpected good news."

The findings suggest that the vaccine is likely to provide meaningful protection against infections caused by subclade K, particularly against severe outcomes such as hospitalization.

This news comes amid a challenging flu season, with subclade K driving the majority of cases across the U.S. and globally.

U.s. Centers for Disease Control and Prevention (CDC) data for Week 52 indicate H3N2 accounts for nearly all influenza activity so far, with subclade K comprising over 90% of characterized H3N2 viruses.

Vaccine experts at the CDC emphasize that vaccination remains the best tool available, even against mismatched strains. Furthermore, there is an ample supply of flu shots offered at clinics and pharmacies in 2026

This preprint has not yet undergone peer review, but preliminary data align with observations from public health agencies monitoring the season.