Rare Form of Drug-Resistant Seasonal Flu Detected in U.S.

June 13, 2024 – The CDC has detected two human cases of influenza that are resistant to the most commonly used antiviral flu drug, known by the brand name Tamiflu.

Laboratory tests showed that two U.S. samples of influenza virus carried two major mutations that came from the most common version of seasonal flu called influenza A (H1N1). 

Seasonal flu viruses are different than the avian or bird flu viruses that have recently made headlines for being found in cattle and humans. 

The Tamiflu-resistant H1N1 virus has two mutations called I223V and S247N. The U.S. cases were identified as part of the CDC’s annual analysis of U.S. and international flu samples. 

A total of 101 samples were identified as the “dual mutant” version of H1N1, and the samples originated from a total of 15 countries spanning five continents. The earliest sample was collected in Canada in May 2023, and the latest samples were collected in Europe during January and February 2024. The Netherlands had the most samples of the dual mutant flu virus, with 30.

The mutated version of the virus was not resistant to other, less-used antiviral drugs, according to the report in the CDC’s journal Emerging Infectious Diseases. The dual mutant form of the flu virus was first reported earlier this year by scientists in the journal The Lancet Microbe, who were studying flu samples from Hong Kong collected in October 2023. 

Flu antiviral drugs can reduce the severity of the flu and how long symptoms last. Some studies have shown a link between taking these drugs and a lower risk of death for adults hospitalized with the flu. The drugs work best when a sick person begins taking them within one to two days of symptoms starting.

Since October 2023, there have been an estimated 35 million cases of flu in the U.S., leading to 390,000 hospitalizations and 24,000 deaths, according to the CDC’s weekly FluView publication. Among flu specimens tested by public health laboratories in the U.S. this past flu season, 76% were types of Influenza A, with the remainder being Influenza B.

Among hospitalized adults, COVID-19 remains slightly more deadly than the flu, according to an analysis recently published in the Journal of the American Medical Association. The death rate among adults hospitalized in 2023-2024 with COVID-19 was 5.7% at 30 days, compared to 4.7% at 30 days for adults hospitalized with flu during the same time period. The authors noted that twice as many hospitalizations occurred for COVID-19, compared to flu.