The Centers for Disease Control and Prevention (CDC) has identified two human instances of influenza that exhibit resistance to the predominant antiviral medication, Tamiflu.

Analyses revealed that these U.S. influenza virus samples harbored two significant mutations, originating from the prevalent strain of seasonal flu, influenza A (H1N1).

Seasonal influenza viruses differ from avian or bird flu strains, which have recently garnered attention for their presence in both livestock and humans.

The Tamiflu-resistant H1N1 strain contains two mutations, I223V and S247N. These U.S. cases were detected through the CDC’s annual scrutiny of both domestic and international flu specimens.

In total, 101 samples were identified as the “dual mutant” H1N1 variant, with origins spanning 15 countries across five continents. The earliest specimen was collected in Canada in May 2023, while the latest were gathered in Europe during January and February 2024. The Netherlands reported the highest number of these dual mutant samples, totaling 30.

According to the CDC’s journal Emerging Infectious Diseases, this mutated virus strain showed no resistance to other, less frequently used antiviral medications. The dual mutant variant was initially reported earlier this year by researchers in the journal The Lancet Microbe, who examined flu samples from Hong Kong collected in October 2023.

Antiviral medications for the flu can alleviate symptom severity and duration. Some research has suggested a correlation between these drugs and a reduced mortality risk in adults hospitalized with influenza. These medications are most effective when administered within one to two days of symptom onset.

Since October 2023, the U.S. has seen an estimated 35 million flu cases, resulting in 390,000 hospitalizations and 24,000 fatalities, as per the CDC’s FluView publication. Among flu samples tested by U.S. public health laboratories this past flu season, 76% were Influenza A, with the remainder being Influenza B.

A recent analysis in the Journal of the American Medical Association indicated that COVID-19 remains marginally more lethal than the flu among hospitalized adults. The 30-day mortality rate for adults hospitalized with COVID-19 in 2023-2024 was 5.7%, compared to 4.7% for those hospitalized with influenza during the same period. The study also noted that COVID-19 hospitalizations were twice as frequent as those for the flu.