(Reuters) - Two studies showing how scientists mutated the H5N1 bird flu virus into a form that could cause a deadly human pandemic will be published only after experts fully assess the risks, the World Health Organization (WHO) said on Friday.

Speaking after a high-level meeting of flu experts and U.S. security officials in Geneva, a WHO official said an deal had been reached in principle to keep details of the controversial work secret until deeper risk analyses could be carried out.

"There is a preference from a public health perspective for full disclosure of the information in these two studies. However there are significant public concerns surrounding this research that should first be addressed," said Keiji Fukuda, the WHO's assistant director-general for health security and environment.

The WHO called the meeting to break a deadlock between scientists who have studied the mutations needed to make H5N1 bird flu transmit between mammals, and the U.S. National Science Advisory Board for Biosecurity (NSABB), which wanted the work censored before it was published in scientific journals.

Biosecurity experts fear mutated forms of the virus that research teams in The Netherlands and the United States independently created could escape or fall into the wrong hands and be used to spark a pandemic worse than the 1918-19 outbreak of Spanish flu that killed up to 40 million people.

WHO spokesman Gregory Hartl said that because of these fears, "there must be a much fuller discussion of risk and benefits of research in this area and risks of virus itself."

But a scientist close to the NSABB who spoke to Reuters immediately after the decision said the board was deeply "frustrated" by the situation.

The only NSABB member attending the meeting was infectious disease expert Paul Keim of Northern Arizona University and he "got the hell beat out of him," the source said.

"It was a closed meeting dominated by flu people who have a vested interest in continuing this kind of work," he added.

The WHO said experts at the meeting included lead researchers of the two studies, scientific journals interested in publishing the research, funders of the research, countries who provided the viruses, bioethicists and directors from several WHO-linked laboratories specializing in influenza.

HIGH FATALITY RATE

The H5N1 virus, first detected in Hong Kong in 1997, is entrenched among poultry in many countries, mainly in Asia, but so far remains in a form that is hard for humans to catch.

It is known to have infected nearly 600 people worldwide since 2003, killing half of them, a far higher death rate than the H1N1 swine flu which caused a flu pandemic in 2009/2010.

Last year, two teams of scientists - one led by Ron Fouchier at Erasmus Medical Center and another led by Yoshihiro Kawaoka at the University of Wisconsin - said they had found that just a handful of mutations would allow H5N1 to spread like ordinary flu between mammals, and remain as deadly as it is now.

This type of research is seen as vital for scientists working to develop vaccines, diagnostic tests and anti-viral drugs that could be deployed in the event of an H5N1 pandemic.

In December, the NSABB asked two leading scientific journals, Nature and Science, to withhold details of the research for fear it could be used by bioterrorists.

They said a potentially deadlier form of bird flu poses one of the gravest known threats to the human population and justified the unprecedented call to censor the research.

The WHO voiced concerns, and flu researchers from around the world declared a 60-day moratorium on January 20 on "any research involving highly pathogenic avian influenza H5N1 viruses" that produce easily contagious forms.

Dr. Bruce Alberts, editor-in-chief of the journal Science, said it is now likely the paper submitted to Science and to the journal Nature will be published in full.

Alberts said it is still not clear how the scientists in Geneva plan to handle biosafety issues mentioned by the group, and it is still not clear when the papers will be published, but it will likely not be years.

"I hope this does not cause the world governments and WHO to stop working on this problem," Alberts said of any potential fallout from the decision at a news briefing at the American Association for the Advancement of Science meeting in Vancouver.

When asked how the journal is safeguarding copies of the as-yet-unpublished paper, he said it is in a lock

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