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Dr Peter Horby, senior clinical research fellow at the Oxford University Clinical Research Unit in Hanoi, Vietnam, said avian flu strains have been around for more than a decade, but "have not progressed any further down the path towards a pandemic virus".
He added: "Limited human-to-human transmission of H7N9 virus is therefore not surprising, and, like H5N1, H7N7, and H3N2v, does not necessarily represent the early stages of a trajectory towards full adaptation to humans."
Dr Horby said, however, that intensive surveillance was needed, including of the dose of virus needed to infect people and the role of repeated exposure to the strain.
As of the end of June, 133 cases of avian flu had been reported, leading to 43 deaths.
Most cases have been among people visiting markets selling live birds or among those who had contact with live poultry in the seven to 10 days before becoming ill.
The latest study examined the case of a 60-year-old father who regularly visited a live poultry market and became ill five to six days after his last visit in March.
He was admitted to hospital and died of multiple organ failure on May 4. His 32-year-old daughter, who was previously healthy, looked after him before he was admitted to intensive care.
The daughter developed symptoms six days after her last contact with her father and was admitted to hospital. Despite intensive care treatment, she died of multiple organ failure on April 24.
Follow-up investigations revealed almost genetically identical virus strains from each patient, suggesting transmission from father to daughter.
A deadly strain of avian flu may have passed between people for the first time, experts believe.
The avian influenza A (H7N9) virus is thought to have been transmitted between father and daughter in eastern China, according to research published online by the British Medical Journal (BMJ).
The findings provide the strongest evidence yet of H7N9 transmission between humans, but its ability to transmit itself is "limited and non-sustainable," the experts believe.Researchers presented evidence on the strain, which emerged in eastern China in February.