A deadly strain of avian (bird) flu appears to have passed between people for the first time.

The avian influenza A (H7N9) virus was identified in eastern China recently. As of the end of June, over 130 cases had been reported and 43 people had died.

In most cases, those affected had visited live poultry farms or had been in close contact with live poultry seven to 10 days before the illness took hold.

However, scientists have now found a ‘family cluster' involving two patients - a father and daughter - who were both infected with the virus in March.

The father, who was 60 years old, was a regular visitor to a live poultry farm. He became sick five to six days after his last visit to the farm and was admitted to hospital on March 11.

He was transferred to intensive care on March 15 when his condition deteriorated, but eventually died of multi-organ failure on May 4.

His daughter, who was described as a healthy 32-year-old, had not been exposed to live poultry before becoming ill. However, she did provide bedside care to her father in hospital before he was admitted to intensive care.

The daughter developed symptoms some six days after her last contact with her father. She was admitted to hospital on March 24 and moved to intensive care four days later. She also died of multi-organ failure on April 24.

According to the scientists, ‘two almost genetically identical virus strains' were isolated from the two patients. This suggests transmission of the virus from father to daughter.

Following the deaths, over 40 close contacts of both the father and the daughter were interviewed and tested for the virus. Among these, one displayed mild signs of illness, but all the contacts tested negative for the virus.

The scientists believe that the reason for the family cluster of deaths is down to the virus being ‘transmitted directly from the index patient to his daughter'.

However, they emphasised that the virus ‘has not gained the ability to transmit itself sustained from person to person efficiently'.

"To our best knowledge, this is the first report of probable transmissibility of the novel virus person to person with detailed epidemiological, clinical, and virological data. Our findings reinforce that the novel virus possesses the potential for pandemic spread," the Chinese team said.

Details of their findings are published in the British Medical Journal. In an accompanying editorial, scientists from the London School of Hygiene and Tropical Medicine based in Bangkok comment that some transmission between humans ‘is not surprising', but ‘does not necessarily indicate that the virus is on course to develop sustained transmission among humans'.

However, the study ‘does provide a timely reminder of the need to remain extremely vigilant'.

"The threat posed by H7N9 has by no means passed," they said.