So far, most reported cases appear to be of people who have visited live poultry markets, or who were in close contact with live birds in the days leading up to their illness.

A study led by a world expert in avian flu that was published in Nature last month suggests that H7N9 viruses have global threat potential .

Although there is currently no evidence of sustained spread of H7N9 among humans, some experts believe it is just a matter of time before the virus acquires the ability to do so.

The emerging avian influenza A (H7N9) virus was recently identified in Eastern China. Reports up to the end of June 2013 show 133 have so far been infected, resulting in 43 deaths.

However, the Chinese scientific and public health team that investigated the case conclude that since no outbreak occurred, the ability of H7N9 to transmit itself is “limited and non-sustainable”.

The event concerns a father who most likely contracted the virus from live birds, and his daughter, who likely caught it while caring for him in hospital.

This week the BMJ publishes the strongest evidence yet of probable person-to-person transmission of the new bird flu virus H7N9 in a family cluster in Eastern China, where the virus first emerged earlier this year.

For their study, Ming-Hao Zhou, director of Jiangsu Provincial Center for Disease Control and Prevention in Nanjing, and colleagues:

Interviewed 43 relatives and close friends of a father and daughter who both caught the H7N9 virus and eventually died from it.

The father and daughter both lived in the same household in Eastern China.

The researchers weren’t able to speak to the patients themselves because they were too ill.

The father

The 60-year-old father fell sick first. He was a frequent visitor to a market with live animals and became ill five to six days after his last visit, when he had bought some live quails. He was taken to hospital on 11 March but his symptoms worsened and he was transferred to intensive care. He died a few weeks later.

The daughter

The 32-year-old healthy daughter, who reportedly had no contact with live poultry, tended to her father at his bedside in hospital before he went into intensive care. It appears she did so without wearing any protection.

The daughter became ill six days after last being in contact with her father. She was taken into hospital on 24 March, and after a spell in intensive care, also died of organ failure a few weeks later.

The researchers concluded that the virus passed from father to daughter because genetic tests on the strains of H7N9 they were infected with showed they were nearly identical.

None of the 43 family and close contacts interviewed tested positive for the virus.

The researchers also tested samples from poultry cages and water at two local animal markets, and from swans near where the father and daughter lived. They isolated one strain of the virus but genetic testing showed it was not the same as the one that infected the two patients.

They conclude however, that:

“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.”

Zhou says while the evidence shows the virus passed from person to person in this cluster, the infection was “limited and non-sustainable as there is no outbreak following the two cases.”

In an accompanying editorial, James Rudge and Richard Coker from the London School of Hygiene and Tropical Medicine, based in Bangkok, say while this study fails to show H7N9 has moved a step closer to being able to pass easily among humans, there is still reason to be concerned because there are some features of the virus that suggest it has the potential to become a global threat.

We need to remain extremely vigilant, they urge, because “the threat posed by H7N9 has by no means passed.”

Written by Catharine Paddock PhD