Analysis of the new avian influenza A (H7N9) discovered in China earlier this year suggests that the virus was probably transmitted from human to human.
The H7N9 strain was previously understood to spread solely from birds to humans.
This is the first case where it appears to have spread between humans, according to a report published by the British Medical Journal (BMJ).
The BMJ report focused on two patients, a father and his daughter, with H7N9 virus infection in China in March 2013.
Two samples taken from the patients appeared almost identical. That suggest that the father, who contracted the virus after being exposed to a live poultry market some months earlier, transmitted it directly to his daughter. Both patients have since died from multiple organ failure.
No outbreaks have been reported as a result of the two patients becoming infected.
Researchers say that there is no evidence to suggest that the virus in its current from is transmittable between people efficiently or on a large scale. However there is concern that if the virus mutates to spread easily between people that a pandemic may follow.
Scientists say it is vital to prepare for the threat and they are therefore attempting to engineer the virus in high-security laboratories to observe how it may mutate into a more transmittable form. By doing that, researchers and health authorities should be able to better assess how likely it is that a new strain of the virus could become dangerous and how soon they should begin developing drugs and vaccines.
Such work is, however, highly controversial. It has fuelled an international row in the past two years after it was carried out on another threatening bird flu virus called H5N1.
When leading virologists Ron Fouchier and Yoshihiro Kawaoka announced in late 2011 they had found how to make H5N1 into a form that could spread between mammals, the US National Science Advisory Board for Biosecurity (NSABB) was so alarmed that it took the unprecedented step of trying to censor publication of the studies.
The NSABB said it feared details of the work could fall into the wrong hands and be used for bioterrorism.
Meanwhile, the H7N9 bird-flu virus currently appears under control with only three new human cases reported in May, compared with 87 in April and 30 in March. This lull is partly as a result of the closure of several live poultry markets in China as well as the recent warmer weather. However, as winter approaches, many experts fear H7N9 could re-emerge.
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