A new US study has found that the deadly H7N9 strain of bird flu has an "unusual" ability to resist Tamiflu - a key treatment drug - without losing any of its ability to spread in mammals.
The study, carried out by the Mount Sinai School of Medicine in New York, found a mutation in the H7N9 strain of bird flu to be highly resistant to the drug while still sustaining its ability to infect human cells in a laboratory dish.
The results came as two patients who contracted H7N9 in Hong Kong remained under observation in hospital.
The condition of a 36-year-old Indonesian domestic helper has changed from critical to serious, while an 80-year-old man was said to be stable yesterday.
A total of 38 people who came into close contact with the two patients remained in isolation.
The US study, the results of which were published in the journal Nature Communications on Tuesday, analysed a sample from a mainland H7N9 patient in order to examine the strain. While it is widely known that treating flu with antivirals such as Tamiflu can lead to viruses mutating and becoming resistant to such drugs, the study found the H7N9 strain to be unusual.
"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," the study reported.
Nicole Bouvier, who led the study, said: "It's important to emphasise that these H7N9 viruses seem to transmit fairly inefficiently overall. 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."
Researchers said doctors should be prudent in their use of antiviral medicines to treat H7N9 cases, and should consider using drugs other than Tamiflu - such as GlaxoSmithKline's Relenza - where possible.
University of Hong Kong microbiologist Yuen kwok-yung stressed that there was no evidence to suggest the H7N9 strain of bird flu was spreading easily among humans.
A separate study in the US this week suggested that the H7N9 strain would need to undergo multiple mutations before it could become easily transmissible from person to person.
Centre for Health Protection controller Dr Leung Ting-hung said the centre would closely monitor whether the virus had mutated or developed resistance towards treatment drugs.