FluView Week 7: Influenza Remains Elevated & Additional Reports of Antiviral Resistance

 

https://www.cdc.gov/fluview/surveillance/2026-week-07.html

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Three months into this year's influenza season and flu activity remains elevated across much of the country (see map above), although we are seeing a decided shift from influenza A (primarily H3N2) to influenza B (see chart below).

While the big story of this year's flu season has been the emergence of a `drifted' H3N2 subclade K virus, another aspect we've been following have been sporadic reports around the world of reduced oseltamivir effectiveness against H1N1 due to NA-I223V and NA-S247N amino acid substitutions.

A few blogs include:

Eurosurveillance: Expansion of influenza A(H1N1)pdm09 NA:S247N Viruses with Reduced Susceptibility to Oseltamivir, Catalonia, Spain, and in Europe, July to October 2025

China: National Influenza Center Reporting Increased Oseltamivir Resistance in Seasonal H1N1

Taiwan CDC Statement On Increased Antiviral Resistance in Seasonal H1N1 Cases in 2025

Since 2009, global resistance to oseltamivir has run < 1%, although we've seen sporadic community clusters of H275Y mutations (see here, here, and here) and low‑levels of  I223V/S247N reported (see here and here). 

Over the entire 2024-2025 flu season - out of 1697 H1N1 viruses tested - only one carried the NA-I223V and NA-S247N amino acid substitutions.

Reassuringly, during the first 13 weeks of the 2025-2026 flu season (Oct - Dec) the CDC reported zero elevated resistance among the first 193 H1N1 viruses tested (see screenshot from FluView Wk 53).  

On January 10th (Fluview Wk 1), however, the CDC reported 2 (out of now 224) H1N1 viruses (0.2%) tested showed `reduced inhibition by oseltamivir' due to amino acid substitutions NA-I223V and NA-S247N. 

Two weeks ago, in FluView Week 5: Influenza Remains Elevated & Slight Uptick In Antiviral Resistance, we saw that count had increased to 7 reduced inhibition and 3 highly resistant (due to the more worrisome H275Y mutation).

Yesterday's FluView (week 7) - based now on 517 H1N1 isolates tested since October - reports 3 more reduced inhibition (n=10) and a 4th with highly reduced inhibition.

Four A(H1N1)pdm09 viruses had NA-H275Y amino acid substitution conferring highly reduced inhibition by oseltamivir and peramivir. Ten A(H1N1)pdm09 viruses had amino acid substitutions NA-I223V and NA-S247N and showed reduced inhibition by oseltamivir. Two B viruses had amino acid substitution NA- M464T and showed reduced inhibition by peramivir.

The impact of the H275Y mutation on oseltamivir resistance is far greater than I223V/S247N, but the stacking of H275Y with either I223V or S247N can greatly enhance the effect.

Although elevated above the historic baseline, these latest numbers still remain fairly low, and should not affect clinical decisions regarding antiviral treatment. 

But as a trend, they are very much worth noting.  

Which is why we'll be watching future reports to see if these numbers plateau, or continue to rise.