(CNN) -- 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.
In many countries, those objectives have been achieved, according to the letter, and researchers who have permission from their governments to continue this research should do so.
But the United States has been unclear about how long it will be before it issues official guidelines for conditions under which H5N1 transmission research can continue, the letter says. As such, laboratories in the United States and facilities abroad that receive U.S. funding should not proceed with their transmission studies.
Why is there so much controversy about studying this flu? Let's back up.
The H5N1 bird flu can be deadly to people, but as far as scientists know, it does not easily pass from person to person by way of respiratory droplets -- yet. Scientists are trying to figure out the means by which this could happen, so that they could help the world prepare for a possible disastrous pandemic.
The World Health Organization has recorded 355 humans deaths from H5N1 out of 602 cases, although some research has questioned this high mortality rate.
Two groups of scientists independently conducted studies published last year that involved genetically altering a strain of H5N1 influenza. A research group in the Netherlands and a separate group at the University of Wisconsin each created a mutated version of the H5N1 virus, a version is more easily transmitted between mammals than what is found in nature. Both groups used ferrets as test subjects, as these animals closely mimic the human response to the flu.
The results of these studies were not published right away, even though they had been accepted for publication, respectively, by Science and Nature. Concerns were raised in December 2011, partly because of a fear that the research could be misused and become a biological weapon. Other general safety concerns also arose.
Many groups weighed in on what to do with this research. In February 2012, a World Health Organization committee recommended that the two controversial studies be published in their full form. The National Institutes of Health agreed in April.
Finally, the Nature study was published in May, led by University of Wisconsin-Madison researcher Yoshihiro Kawaoka. The Science study, which came out in June, was led by Ron Fouchier at the Erasmus Medical Center in Rotterdam, Netherlands.
While the Nature study looked at how a bird flu virus could become airborne through mutations and re-assortment with other viruses, the Science study suggests mutations alone could do the trick. It would take between five and nine mutations for the H5N1 flu to become airborne, scientists said, which is a low range.
"It's so easily mutated, so the risk exists in nature already, and not doing the research is really putting us in danger," Kawaoka said at a press conference Wednesday.
Kawaoka cannot continue his research in Wisconsin, funded by the National Institutes of Health, pending further guidance from the United States. Fouchier also receives some NIH funding, but the rest is supported by the European Union and other organizations, so his group can continue studying avian flu using those non-U.S. resources.
At Fouchier's facilities in the Netherlands, employees who work on avian flu wear "moon suits" and there's always a barrier between them and the virus, Fouchier said. Workers are also vaccinated against avian flu, although it is hard to find a company to produce the vaccine; the last vaccination was a year ago.
Fouchier's group is not restarting experiments immediately, but probably within the next few weeks, he said. His group will attempt to nail down exactly how many mutations - and which - are sufficient to make the H5N1 avian flu virus airborne, and whether these particular mutations can also make other bird flu viruses airborne.
Avian flu strains from Indonesia and Vietnam has been studied in the context of mutations so far. But, says Fouchier, "there are other genetic lineages of H5N1 in Egypt, in China, for instance, that we would like to test whether also, in these countries, viruses may emerge with an airborne transmission phenotype."