The National Science Advisory Board for Biosecurity (NSABB) set off a furious debate in the scientific and public health communities in December when it asked the journals Nature and Science to censor two studies on new strains of the H5N1 virus that may make it more easily transmissible in people.

"The potential of this pathogen, in theory, exceeds anything else I can imagine," Paul Keim, acting chair of the NSABB, told Reuters in an e-mail.

Keim explained his personal decision to support censorship in this case in a commentary published on Tuesday in mBio, the journal of the American Society for Microbiology. The panel also published an explanatory piece in Nature and Science.

The panel cited fears that mutant versions of the H5N1 virus created by scientists at Erasmus Medical College in the Netherlands and the University of Wisconsin-Madison, could accidentally escape the lab or be used as a devastating form of bioterrorism.

The censorship decision was a first for the panel, and it drew scathing criticism from many researchers who say withholding the information will set scientists back in the search for potential treatments and hamper public health efforts to track the virus.

The researchers involved in the studies have agreed to a 60-day suspension of their work to allow governments and public health agencies to debate how it should best be handled. A meeting on the subject is scheduled for mid-February at the World Health Organization in Geneva.

Keim, who chairs the microbiology department at Northern Arizona University, said the panel considered evidence that bird flu kills about half the people it infects, a much higher mortality rate than the devastating 1918-19 outbreak of Spanish flu that killed up to 40 million people.

Making this deadly virus capable of easy transmission in people was "sobering," Keim wrote in mBio.

"A pandemic by such a pathogen could reasonably be concluded to cause such devastation that it should be prevented at all costs."

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The NSABB was formed after a series of anthrax attacks in the United States in 2001. It advises the Department of Health and Human Services and other federal agencies about "dual use" research that could serve public health but also be a potential bioterror threat.

The National Institutes of Health, which funded some of the research, agreed with the panel's assessment and made non-binding recommendations to Science and Nature to withhold key elements of the work.

Writing in Nature and Science, the NSABB panel said the "unprecedented recommendation" took into account the potential benefits of publishing the works and the harms that could follow. "Our concern is that publishing these experiments in detail would provide information to some person, organization or government that would help them to develop similar mammal-adapted influenza A/H5N1 viruses for harmful purposes."

"We believe that as scientists and as members of the general public, we have a primary responsibility 'to do no harm'," the panel wrote.

First detected in 1997 in Hong Kong, H5N1 has devastated duck and chicken flocks in Cambodia, China, Egypt, India, Indonesia and Iran, and has reached the Middle East and Europe through wild birds.

So far, the virus cannot jump easily from person to person through airborne droplets, but scientists have been warning for years it could mutate into a deadlier flu strain.

Experiments done by Yoshihiro Kawaoka of the University of Wisconsin-Madison and The University of Tokyo, and Dr. Ron Fouchier from the Erasmus Medical Center in Rotterdam showed how it could happen. With just a few mutations, the teams made the virus easily transmissible between ferrets, which are used in the "lab to predict how flu viruses will behave in people.

In Fouchier's experiments, the virus proved to be highly transmissible and highly deadly in ferrets. In Kawaoka's experiment, the mutated virus was highly transmissible but not as deadly, raising questions about why the panel decided both papers should be censored.

In Nature, Keim answered questions about the Kawaoka paper in detail, saying that although the virus was not as deadly as Fouchier's, there were still risks.

"The fact that Kawaoka's specific virus and mutations might not be the feared H5N1 pandemic strain is not the point. It is that this laboratory created a virus that has now bypassed apparent barriers to evolution in the wild," Keim wrote.

"If this virus were to escape by error or by terror, we must ask whether it would cause a pandemic. The probability is unknown, but it is not zero."

Vincent Racaniello of Columbia University, who wrote a commentary in mBio opposing the panel's decision, points out that it is still not known whether the ferret-adapted virus in either experiment is lethal or transmissible among humans.

He said altering viruses so that they can live in lab animals is often used as a strategy for making a virus weaker and less suitable for living in people.

And Racaniello worries about the precedent of publishing parts of studies without revealing how the work was done, making it harder for other scientists to validate the work.

Keim said in an e-mail he agrees that ferrets are not 100 percent predictive of human disease, but that they are still the best model scientists have for predicting whether a flu virus will be capable of infecting people.

"To gamble that this model is wrong on this issue is very dangerous," Keim said. "Why would we risk a global pandemic saying that our best model is wrong?"

(Reporting by Julie Steenhuysen; Editing by Michele Gershberg and Paul Simao)

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