There's no guarantee the H5N1 avian flu virus would become less deadly to people if it triggers a pandemic, a new report from the World Health Organization warns.

A group of eminent influenza scientists gathered by the WHO last month concluded there is no reason to believe that the virus, which kills roughly 60 per cent of people who become infected, would become any milder if it evolves to become a pandemic strain.

The report, based on that meeting, cautions governments against spending a lot of money to stockpile existing H5N1 vaccines. It also reveals that a low level of H5N1 viruses found in wild and domestic birds appears to be naturally resistant to oseltamivir, the main flu drug being stockpiled against a future pandemic.

The WHO scientist who convened the meeting cautioned, however, that the question of how lethal an H5N1 pandemic might be is the scientific equivalent of a black hole.

Modern science has never seen a flu virus as nasty as H5N1 and there is no way of knowing if the virus can become easily transmissible among people or what an H5N1 pandemic would look like.

"It's one of those things that you hate to conjecture," said Michael Perdue, an avian influenza expert and scientist with the WHO's global influenza program.

"We just don't know enough about this virus, a whole new subtype for humans…. If a new H5 enters, it could be more lethal than anything we've ever seen in history. (But) who knows?

"I think it's anybody's opinion because it's just so completely unknown."

Pandemic effects difficult to predict

The report noted some modelling studies suggest a highly lethal virus could not spark a pandemic because people who fall gravely ill aren't walking about transmitting flu to others. "All such matters remain difficult to predict," the report concludes.

It also questions the wisdom of stockpiling current versions of H5N1 vaccine for later use, saying there is little evidence that a vaccine against one variant of the virus will induce a good immune response against even other currently circulating H5N1 viruses, let alone future ones.

"We still don't have the data that would say that a pre-pandemic vaccine based on one H5N1 strain would completely protect against another," Perdue said from Geneva on Thursday.

"Although one would hope it would mitigate the effects some, we don't have the data to show it."

The United States and Switzerland are stockpiling current H5N1 vaccines. And several other countries — Singapore and Britain among them — are reported to be in negotiations with vaccine maker GlaxoSmithKline to buy bulk lots of its H5N1 vaccine for stockpiling purposes.

Genetic susceptibility

The report's authors also said some people may have a genetic predisposition to bird flu infection, which could explain why not everyone gets it.

The conclusion is based on an analysis of seven deaths among an extended family in Indonesia last May that suggested genetic factors may affect how susceptible humans are to H5N1.

The H5N1 strain of bird flu does not spread easily from animals to humans or between people — a condition for a pandemic strain.