Doctors in Norway discovered that there is a drug-resistant influenza strain which might be linked with patients who suffer more severe flu symptoms. The strain has spread all around the world this year and, as the doctors say, it needs further study to calculate its virulence.

Researchers from the Norwegian Institute of Public Health in Oslo and the U.K.'s Health Protection Agency said that the form of the H1N1 seasonal flu virus can’t be treated by Roche Holding AG’s Tamiflu and it seems to be able to cause pneumonia and sinusitis. On the other hand, the H1N1 strain might not be possible to do this.

Yet, the authors said that this study still needs further investigations, because they had only tracked less than 300 patients. The study, released online today, will be published in the February edition of Emerging Infectious Diseases by the U.S. Centers for Disease Control and Prevention.

Tamiflu was reportedly the best selling antiviral, but its evading influenza strain has been reported too in 50 countries from six continents since it had been announced to be the best fighter against flu. These results made the doctors to consider other medicines, such as GlaxoSmithKline Plc's Relenza.

Influenza and its additional causes, such as pneumonia, cause 250,000 to 500,000 deaths annually, according to WHO. The CDC said in a statement released on December 19 that the doctors from the United States should prescribe Relenza to their patients.

They also added that a combination of Tamiflu and rimantadine could work to cure the influenza.