Flu under scrutiny: Changes in viruses don't mean the end of drug effectiveness

Influenza A accounted for over 97% of detections reported in European laboratory surveillance during the 2025–2026 season. A new ECDC report describes the dominant virus variants and rare variants associated with reduced susceptibility to antiviral drugs.

The results don't mean that flu treatment is no longer effective. It's important to distinguish between the detection of a specific genetic change and confirmation of resistance in a laboratory test, and even more so from the ineffectiveness of treatment in a specific patient.

What does the ECDC report show?

The analysis, published on November 6, covers data from 30 European Union and European Economic Area countries. It covers the period from the 40th week of 2025 to the 33rd week of 2026, i.e. from the turn of September and October to August.

Total reported 256,967 influenza virus detectionsThis is not a total number of cases in Europe, but rather a result of laboratory reporting. The summary does not describe the current epidemiological situation in November.

Type A clearly dominated. Type B accounted for a small proportion of reports, and in the remaining cases the virus type was not specified.

What changes have been detected in viruses?

Among influenza A viruses, subtypes were monitored A(H1N1)pdm09 and A(H3N2)Genetic analyses indicated the dominance of the D.3.1.1 subgroup in the former and the K subgroup in the latter.

They constituted approximately 95% and 92% of viruses covered by a given genetic analysisThese values should not be applied to all reported infections.

Such designations help scientists track the evolution of viruses. However, the mere presence of a new subgroup does not prove that it causes more severe disease or that vaccines do not provide protection.

Flu medications. What do the test results mean?

In the case of A(H1N1)pdm09, genetic changes associated with reduced viral inhibition by oseltamivir was detected in 2.4 percent of the samples evaluated. Changes associated with significantly reduced inhibition were found in a further 0.2 percent.

For A(H3N2) the corresponding percentages were around 0.2% and below 0.1%. Changes related to reduced sensitivity to baloxavir occurred sporadically in both subtypes — at a level of approximately 0.1%.

No reduced susceptibility to the drugs assessed was found in the tested influenza B viruses.

These are percentages among samples subjected to specific tests, not percentages of patients whose treatment failed. Individual analyses included different numbers of viruses.

Why monitor treatment sensitivity?

Regular testing helps identify changes that may impact treatment recommendations. It also helps determine whether specific viral characteristics remain rare or are becoming more widespread.

Antiviral treatment and vaccination serve different purposes. Detecting changes in drug susceptibility is not an assessment of vaccine effectiveness and should not be equated with it.

The report primarily serves as a basis for continued virus surveillance. It does not provide a basis for declaring widespread influenza resistance to treatment or for independently changing recommended therapy.