GlaxoSmithKline says it is ready to up its supply of the flu drug Relenza in the USA after health officials warned that a strain of influenza circulating in the country this winter is resistant to Roche’s rival drug Tamiflu.

The Centers for Disease Control and Prevention has issued a special advisory telling doctors that 49 out of 50 influenza A H1N1 samples it has tested have resistance to Tamiflu (oseltamivir), noting that the strain has been observed mostly in Hawaii, Massachusetts and Texas. Last year, just 11% of samples tested were resistant to the Roche drug.

CDC director Julie Gerberding said that "this is probably actually not going to affect very many people because we don't use a lot of antiviral drugs in our country”. She added that it is still very early in the season and “there is very little influenza out there." Tamiflu, which may still be used to treat the influenza A H3N2 and influenza B strains, comprises 80% of the US national stockpile, with GSK’s Relenza (zanamivir) making up the rest.

Nevertheless, the CDC is recommending that doctors prescribe Relenza or Tamiflu plus rimantadine, in cases of suspected infection with the H1N1 strain. Roche says that the CDC advisory is based on "a small sample in a limited number of states, and Tamiflu is showing good activity against other circulating viruses."

GSK noted that “based on early and limited data” from the current flu season that has detected a significant increase in the H1N1 viruses that are resistant to Tamiflu, it is "prepared to respond to public health needs for Relenza". All tested influenza virus subtypes have shown no resistance to Relenza, the firm said, adding that it has sufficient supplies for the 2008-2009 flu season.

However the CDC stressed that Relenza is not approved for people under seven years old, and people with asthma are also advised not to use the drug.