Preliminary results have shown that antiviral drug Tamiflu is not always effective.

Preliminary results from European public health specialists on Monday identified significant resistance to the drug Tamiflu, the world’s most widely purchased influenza antiviral medicine.

The European Centre for Disease Prevention and Control said that while Tamiflu could still provide benefits, 13% of samples of the H1N1 seasonal flu virus affecting Europe tested last November and December – mostly in Norway – contained a mutation associated with high levels of resistance.

The preliminary data, based on 148 samples collected from 10 European countries, is the first important indication of resistance to Tamiflu, which has been purchased in large quantities by governments around the world over the past three years as a defence against a flu pandemic.

Previous studies have indicated far lower resistance, with only about 0.4% of seasonal viruses rendering Tamiflu treatment less effective. There have been four cases of drug resistance among the patients with current variants of H5N1 treated to date; the strain has infected 353 people worldwide since 2003.

The news is unfortunate for Roche, the Swiss pharmaceutical group, which has sold 220 million treatments of Tamiflu in recent years for pandemic flu stockpiles to 85 countries.

“The new European Medicines Agency (EMEA) guidance is a timely reminder that we need to constantly review our pandemic preparedness plans,” said Professor Albert Osterhaus, Professor of Virology, University of Rotterdam, Netherlands, and Chairman of the European Scientific Working Group on Influenza (ESWI).

“The issue of viral resistance is of concern to influenza experts and I welcome the EMEA guidance that the availability of more than one antiviral would be useful. On reviewing pandemic strategies, Governments should seriously consider including more than one antiviral.”

GlaxoSmithKline, the UK-based pharmaceutical group that markets Relenza, an alternative to Tamiflu, has welcomed this. It has invested €1.7 billion (£1.26bn) in expanding its global manufacturing and research capabilities in both vaccines and antivirals and in ensuring the continuity of critical business operations and processes, to safeguard the continued supply of critical medicines.