London: With the new bird flu that has emerged in China, drug resistance has been detected in patients infected.
The H7N9 virus became resistant to Tamiflu in three out of 14 patients treated with antiviral drugs at Shanghai Public Health Clinical Centre, the BBC reported.
The researchers, writing in the Lancet, said resistance emerged with "apparent ease" and was "concerning".
There have been no new cases of the infection for more than two weeks.
Doctors analysed the virus in 14 patients. All had pneumonia and half needed ventilation to keep them alive.
Treatment with antivirals reduced levels of H7N9 in most patients and led to an improvement in symptoms.
Yet the treatment failed in three patients. Genetic testing of the virus in these patients showed it had acquired the mutations needed to resist the drugs.
The doctors believe that in at least one patient the emergence of resistance was a direct consequence of treatment with Tamiflu.
Such antiviral drugs are the only way of treating the infection.
The researchers said: "The apparent ease with which antiviral resistance emerges in H7N9 viruses is concerning, it needs to be closely monitored and considered in future pandemic response plans."