The discovery means that unlike seasonal flu strains, which often become less transmissible when they develop resistance to drugs like Roche's Tamiflu, the new H7N9 bird flu does not lose any of its spreading potential with drug resistance.
While this does not make H7N9 any more likely to develop into a human pandemic, researchers said it means doctors should be prudent in their use of anti-viral medicines to treat H7N9 cases, and consider using drugs other than Tamiflu, such as GlaxoSmithKline's Relenza, where possible.
"It's important to emphasise that these H7N9 viruses seem to transmit fairly inefficiently overall," said Nicole Bouvier, who led the H7N9 study which was published in the journal Nature Communications.
"But what was surprising about our study was that the drug-resistant virus was no less efficient than the drug-sensitive one. Usually what we see with influenza, is that resistance... also confers a fitness disadvantage on the virus."
H7N9 bird flu emerged earlier this year in China and has infected at least 139 people so far in China, Taiwan and Hong Kong, killing 45 of them.
Experts say there is no evidence as yet of any easy or sustained human-to-human transmission of H7N9. But an early scientific analysis of probable transmission of the new flu from person to person, published in August, gave the strongest proof yet that it can at times jump between people and so could potentially cause a human pandemic.
A separate team of researchers in the United States said this week that while it is not impossible that H7N9 could become easily transmissible from person to person, it would need to undergo multiple mutations to do that. So scientists around the world are keeping a watchful eye, on alert for any sign the virus might develop such potential.
Antiviral drugs such as Roche's Tamiflu, known generically as oseltamivir, are useful for treating flu infections, but human cases of Tamiflu-resistant H7N9 were found fairly swiftly after the current outbreak began.
For her study, Bouvier's team at the Mount Sinai School of Medicine in New York analysed a mutated H7N9 virus taken from a sample from an infected patient in China, examining its resistance to drugs and its infectivity.
They found it was highly resistant to Tamiflu, but also that it still had the ability to infect human cells in a laboratory dish, and spread between laboratory animals just as efficiently as its non-mutated counterpart.
"This is unusual, as it is known that when seasonal influenza viruses gain resistance to drugs, it usually happens at a cost to the virus, the cost being a reduced ability to transmit between hosts and to grow within them," they wrote.
And since it is known that treating flu with antivirals can lead to drug resistance "this study further underscores the need of prudent use of antivirals in H7N9 influenza infections."