Switzerland’s Roche has cited data from a new study from the World Health Organisation which confirms a low frequency of resistance to the firm’s antiviral drug Tamiflu over the last three influenza seasons.

The firm said that the data, published by the Neuraminidase Inhibitor Susceptibility Network in the WHO’s Weekly Epidemiological Record, shows that resistance of around 0.3% to Tamiflu (oseltamivir) was seen during the influenza seasons (2003-2006) in which there had been substantial use by 35 million patients in Japan , the highest use in any market. The company noted that this level of resistance is extremely low compared to rates of 65% seen in Japan with another, much older, antiviral, amantadine.

"These results confirm that the potential for the development of resistance to Tamiflu is very low, even when used extensively in the management of seasonal influenza," said David Reddy, pandemic task force leader at Roche. He added that “this provides reassurance to the scientific community that since the introduction of Tamiflu in 1999, the levels of resistance have remained similar to those seen in the clinical development programme."

Roche has repeatedly spoken out against speculation that resistance to Tamiflu is on the rise, especially in patients infected with H5N1 avian flu. The company acknowledged that the possible development of antiviral resistance “is of concern for pandemic planning and preparedness but stated that to date, there have only been three documented cases of Tamiflu resistance to bird flu. In one case, the prophylactic (75mg daily) rather than the treatment dose (75mg twice daily) was given to a sufferer already exhibiting clinical symptoms, “thus under-dosing the patient and increasing the risk of resistance,” the Swiss firm added, and once the twice-daily dose was provided, the patient recovered from her illness.

In the other two cases, the recommended dose and duration of oseltamivir was indeed followed, but while one patient received treatment on the second day of illness, the other patient started treatment late, on the sixth day.

Roche added that two further possible cases of resistance of H5N1 to Tamiflu, in Egypt, have been identified and are under investigation. It was the news of this case, in January, that led to the WHO issuing a warning about the possibility of such a scenario, but it also noted that it had no plans to change its recommendation to treat bird flu patients with Tamiflu.