According to recent reports, more than 98 percent of the influenza A viruses which circulate this year is now resistant to common antiviral treatment compared to just 1 percent two years ago.

A study published in The Journal of the American Medical Association concludes that the most important antiviral drug no longer works against this season's most prevalent type of flu, which has mutated into a resistant strain.

“It would not be our message that the medicine doesn't work. There certainly is a resistant type of virus which we are watching closely, but we're early enough in the season where we don't know what this means for the peak of the season,” said Dr. Joseph Bresee, CDC Influenza Division Chief of Epidemiology and Prevention.

The U.S. Centers for Disease Control and Prevention issued an advisory recommending for doctors advising them to treat children with H1N1 with a combination of Tamiflu and another drug, Flumadine, as they found the strain was not resistant to this treatment.

However, only the H1N1 virus is showing signs of Tamiflu resistance, according to the national Centers for Disease Control and Prevention. The other types of flu - influenza B and influenza A H3N2 - can still be treated with Tamiflu, which continues to be the most appropriate treatment. At the same time, all flu viruses can still be treated with GlaxoSmithKline PLC's Relenza antiviral drug or a combination of Roche Holding AG's Tamiflu and a generic drug called rimantadine.

A second study published in The Journal of the American Medical Association has revealed that the Tamiflu-resistant virus poses a significantly health treat against those with weakened immune systems.