It is now five years since the present outbreak of H5N1 avian flu first infected people. Though 379 people have since contracted the virus, of whom 239 have died, it has yet to start a pandemic.

As its name suggests, bird flu remains predominantly an avian disease. While it is very dangerous to humans who catch it, this has happened only rarely, after close contact with infected birds.

This week, however, brought some alarming news. Writing in The Lancet, a Chinese medical team confirmed that a 52-year-old man who contracted H5N1 in Jiangsu province last December almost certainly caught it from his 24-year-old son, who died. It is the best-documented case of human-to-human transmission to date.

That is important because, if this virus is going to start a pandemic, it must first acquire the ability to move readily from person to person. Not enough people are ever going to catch it from birds to constitute a global threat. The Chinese case, like a previous suspected human-to-human incident in Thailand, has thus raised fears that H5N1 might be mutating in worrying fashion, and it was duly reported around the world.

The details of the Lancet study, however, are less troubling than they at first appear. This investigation of this cluster of infections, indeed, is somewhat reassuring because of what it shows has not happened.

First, genetic analysis of virus samples recovered from both men has revealed no substantial mutations from the standard circulating strain. It does not seem to be evolving, yet at least, into a humanised version that can easily set up home in the human respiratory tract.

Furthermore, the scientists found that 91 people had close contact with one or both of the infected men, and yet the virus was passed on only once. This is more good news, confirming that the existing strain is poorly-adapted to people. The father seems to have been extremely unlucky. The case is also encouraging in that it has offered two new leads for medical research, which could prove important in tracking the threat from bird flu and establishing how it is best treated.

The way the virus was passed on only from son to father may reflect their intimate relationship, which probably involved closer contact than with the other people who were exposed. But it could also indicate that they shared a genetic susceptibility to H5N1. While this is far from certain, if it is confirmed it would provide an important clue that could assist efforts to design drugs and vaccines against it.

Another important detail is that the father recovered, after treatment with antivirals and blood plasma from a woman who'd been vaccinated against H5N1 in a clinical trial. The apparent success of this therapy could be significant, as it has been suggested as an emergency means of containing a pandemic in the absence of a vaccine.

Perhaps the most positive aspect of these cases, however, is that we have learnt about them in such detail at all. Not long ago, few international scientists would have expected Chinese officials to have released data about the Jiangsu cluster so freely. The country had a reputation for secrecy about H5N1 cases, and for its reluctance to share virus samples and information.

China now seems to have had a change of heart: the Jiangsu virus was analysed in collaboration with the US Centers for Disease Control, and the results have been published in The Lancet, a journal with wide circulation.

That can only be a welcome development: the hazards posed by avian flu are global and efforts to understand the virus and develop counter-measures will benefit from such openness. H5N1 flu might not yet have triggered a pandemic, and this case does not suggest it is about to. But the virus hasn't gone away, and international surveillance and co-operation is going to be critical to managing its future threat.

Mark Henderson is the Science Editor of The Times

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