“The World Health Organisation considers this to be a very unusual event,” said John McCauley, director of the WHO Collaborating Centre for Influenza at the UK National Institute for Medical Research. “With a 20 per cent mortality rate it is serious but we don’t know whether it is going to spread from human to human.”

Genetic analysis shows that H7N9 has two of the five mutations believed to be required for a flu virus to spread easily through the air between people, said Wendy Barclay, professor of influenza virology at Imperial College London.

But intensive surveillance in China has not provided clear evidence of spread between people – everyone infected so far could have picked up the virus from domestic poultry or wild birds.

Diagnosis has proved difficult because the virus does not kill infected poultry, although detailed blood studies showing its absence in humans who have had contact with infected patients suggests it is not transmitting easily or to large numbers.

The biggest difference between H7N9 and H5N1 – the virulent avian flu strain that has infected 628 people and killed 374 over the past 10 years without transmission between humans – is its effect on birds.

H5N1 has been easier to track because it causes severe disease in domestic poultry, while H7N9 has little or no effect on birds.

Jeremy Farrar, the flu expert who runs the Wellcome Trust Overseas Programme in Vietnam and takes over as head of the Wellcome Trust in October, said: “H7N9 has been found in birds in all 31 Chinese provinces . . . This has to be taken seriously but calmly.”

“What a difference it has been working with China on this, compared with Sars 10 years ago,” Prof Farrar added.

“Their openness in sharing data and samples has been a huge tribute to them.”

Chinese researchers published yesterday in the Lancet the first comprehensive genetic analysis of the origins and evolutionary history of H7N9. This shows the complex way flu viruses rearrange themselves in birds and animals.

H7N9 probably evolved from at least four viral components in ducks and chickens – and has already split into two lineages during the past few months, the Lancet paper says.

Human victims of H7N9 range in age from two to 89, though people affected tend to be primarily middle-aged and elderly men, in contrast to the 2009 H1N1 swine flu pandemic that mainly affected younger people – suggesting that there is no immunity in the population from previous exposure to similar flu viruses. The symptoms are mainly respiratory though in severe cases the patient’s immune system can go into overdrive, causing death.

H7N9 is sensitive to Roche’s Tamiflu, the most widely used anti-flu drug. But many patients have sought medical help too late for the medicine to work and the main weapon, if the virus starts circulating between people, is likely to be a new vaccine, which will have to be developed.