It had been a long established scientific fact that when a flu virus obtained an immunity to particular anti-viral flu medications, they would become less effective in transmission between humans.

Now scientists have found the deadly H7N9 bird flu in China does not lose any of its infectivity when it becomes resistant to commonly used drugs like Tamiflu.

The researchers were quick to add that the drug-resistant H7N9 was not more infectious than in the past. They reiterated that the virus is one of the less transmittable viruses between humans.

H7N9 emerged earlier this year in China and has killed 45 of the 139 people confirmed to have been infected with the virus. Scientists had initially believed H7N9 could not transmit between humans but found cases in August of human-to-human transmission.

A separate study in the United States this week said that it was not impossible for H7N9 to mutate into a form that could be easily passed among humans.