No single drug alone would be adequate to treat all people who would become infected during a worldwide flu pandemic, and nations need to stockpile more than one type of drug, say scientists at the National Institute for Medical Research in Great Britain.

They analyzed samples of H5N1 bird flu that had infected humans and found a mutation that made it resistant to the anti-viral drug Tamiflu. However, the mutation didn't protect the virus from another drug called Relenza, BBC News reported.

The study appears in the journal Nature.

"In order not to be outflanked by the virus, it will be necessary to have stocks of both existing drugs," said research team leader Dr. Steve Gamblin, BBC News reported. "There is a huge imperative to develop further drugs and it is likely a future pandemic will need to be tackled using a three- or four-pronged approach, much as we tackle HIV today."