Drama surrounding research on the deadly H5N1 avian flu continues, as 40 scientists urge work on the virus to continue in countries that have established guidelines on the safety and aims of the research.  The United States is not among them.

This new correspondence, a letter from researchers published Wednesday in the journals Science and Nature, comes after a "voluntary pause" in the research, which scientists announced in January 2012.

"We declared a pause to this important research to provide time to explain the public-health benefits of this work, to describe the measures in place to minimize possible risks, and to enable organizations and governments around the world to review their policies (for example on biosafety, biosecurity, oversight, and communication) regarding these experiments," the letter states.

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Here's what it takes to make a deadly virus transmissible through the air: as few as five genetic mutations, according to a new study.

This research, published in the journal Science, is the second of two controversial studies to finally be released that examines how the H5N1 bird flu virus can be genetically altered and transmitted in mammals. Publication of both studies had been delayed many months due to fears that the research could be misused and become a bio-security threat.

Although these particular engineered forms of H5N1 have not been found in nature, the virus has potential to mutate enough such that it could become airborne.

H5N1 influenza can be deadly to people, but in its natural forms it does not easily transfer between people through respiratory droplets, as far as scientists know. The World Health Organization has recorded 355 humans deaths from it out of 602 cases, although some research has questioned this high mortality rate.

The journals Science and Nature had agreed to postpone the publication of the two studies related to the genetically altered virus.

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