Researchers reported Monday that Tamiflu-resistant influenza A viruses are now spreading widely across the USA, imperiling a pillar of the global pandemic-response stockpile. The most important antiviral drug no longer works against this season's most prevalent type of flu, which has mutated into a resistant strain, researchers reported in the Journal of the American Medical Association.

That drug sold in the U.S. as Tamiflu by F. Hoffmann-La Roche Ltd. was one arrow in a very small quiver of antiviral medicines used to battle influenza, an illness that lands 200,000 Americans in the hospital and kills 36,000 every year, according to the Centers for Disease Control.
Beginning in February 19, resistance to oseltamivir, or Tamiflu, had been identified in 98.5% of U.S. influenza A(H1N1) viruses tested. Some 264 of 268 samples of the H1N1 strain taken this flu season were resistant to Tamiflu.

That’s a startling increase over the 2007-2008 flu season, when resistance was detected for the first time in 12% of H1N1 viruses tested. Each flu season, several types of flu viruses circulate, and various ones can dominate in different regions and times.

However, only the H1N1 virus is showing signs of Tamiflu resistance, according to the national Centers for Disease Control and Prevention. In other words the virus suffered a mutation (a neuraminidase (NA; an enzyme) gene H274Y mutation).

Viruses carrying this mutation have been presumed to be of lower risk and less likely to be transmitted. "However, current widespread circulation of oseltamivir-resistant influenza A(H1N1) viruses associated with typical influenza illnesses and viral pneumonia suggest that these viruses retain significant transmissibility and pathogenicity [ability to cause disease]," the authors write.