And new research, which played a role in reversing the initial ban on full publication of the study, says viruses with two of those changes are already cropping up regularly in nature. That means that if bird flu viruses were able to pick up three specific additional mutations, they might be able to infect human respiratory tracts and trigger a pandemic.
There is no current way to gauge whether that will happen. But the second paper says based on mathematical modelling it must be considered "a potentially serious threat." It estimates that the needed changes could be acquired during infection of a single host — a pig or other intermediary animal or even a person.
"All we can say right now is that it's not unreasonable to expect that if a virus starts three mutations away, that it may be possible to transmit," said Derek Smith, a professor of infectious diseases informatics at Cambridge University and senior author of the modelling paper.
Both studies were published Thursday in the journal Science, seven weeks after a similar — and similarly controversial — paper was published in Nature showing another route H5N1 could take to become a human virus.
The combined material, flu experts say, provides a sharp warning that there doesn't appear to be a biological barrier preventing H5N1 viruses from evolving to be able to spread person to person. Emboldened by the fact that the virus hasn't yet made that leap after years of transmission among birds, some skeptics have claimed H5N1 doesn't have what it takes to become a human flu strain.
Ron Fouchier, the Dutch virologist who led the study that identified the five mutations, said the work underscores how critical it is to look for these mutations — and others that trigger the same functional changes — in H5N1 viruses.
"I do think that once you get to see these receptor binding changes, we have to be really careful," Fouchier, a leading flu expert who works at Erasmus Medical Centre in Rotterdam, said in an interview.
"Because those last changes, as Yoshi showed and we showed, are very easy to get."
Fouchier was referring to Yoshihiro Kawaoka, an influenza virologist at the University of Wisconsin-Madison. Kawaoka published the earlier article in Nature showing that a reassortant or hybrid virus — with one H5N1 gene and seven genes from pandemic H1N1 — could come together and with as few as three mutations acquire the ability to transmit through the air among ferrets, a stand-in for humans in many flu experiments.
H5N1 viruses are endemic in a number of countries in Asia, the Middle Eastern and North Africa. Viruses which already have two of the mutations have been found in 28 countries, mainly in North Africa, the Middle East and Europe, said Colin Russell, lead author on the modelling paper.
An expert panel that advises the U.S. government on biosecurity issues recommended late last fall that the Fouchier and Kawaoka papers be published in abbreviated form only, with key details of the work left out. The National Science Advisory Board for Biosecurity argued that the studies were virtual recipes for turning bird flu into a biological weapon. The U.S. government accepted the recommendation and asked Science and Nature to publish shortened versions of the papers.
Months of wrangling ensued. Flu experts argued those involved in surveillance needed to be on the lookout for H5N1 viruses with these mutations. Some biosecurity experts and other life scientists countered that laboratories should not be in the business of making dangerous viruses more dangerous.
In the end, under pressure to reverse their position, in late March the NSABB voted unanimously to recommend full publication of the papers. One of the key voices on the NSABB arguing against publication said even if the studies are now out, the debate over this type of work is only just beginning.
"The publication of this paper in no way brings this situation to a close," said Michael Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota. "It just reminds all of us that it's only going to get more complicated as additional (scientific) manuscripts come down the pike."
A commentary published with the Fouchier paper suggests others share Osterholm's concern.
The piece, authored by three well-known flu researchers and Dr. Barry Bloom, former dean of the school of public health at Harvard University, argues that future work like this should be limited, with more consideration given to the risks as well as the benefits before the research is undertaken. As well, it recommends that this type of work be restricted to very few labs worldwide — a difficult end to achieve.
Lead author Marc Lipsitch, an infectious disease expert from Harvard, said the world shouldn't take too much reassurance from the fact that the transmissible viruses generated in both the Kawaoka and Fouchier experiments weren't lethal to ferrets. It's impossible to know if that would hold true in people, he argued.
"Saying none of the ferrets which were infected died is — you know, I'd rather hear that than that they all died quickly," Lipsitch said.
"But it's not hugely reassuring. Both because we know that ferrets don't tell the truth about humans every time and because we know that evolution happens and it can happen quickly. And we don't know in which direction it would go."
The World Health Organization plans to hold a meeting to explore the concerns raised by these two studies. Initially scheduled for this summer, the meeting has been pushed into 2013, the WHO point person for this issue said in an interview.
Dr. Keiji Fukuda said researchers will have to think their research through more thoroughly, and the organizations they work for will need to help them anticipate the public's concerns and respond to them.
"If we can move to that kind of scientific culture, then I think a lot of the issues which were raised by these two articles will in fact be addressed. Because these kinds of issues are going to come up, over and over again in the future," said Fukuda, the WHO's assistant director general for health security and environment.
Meanwhile, scientists like Fouchier are eager to resume work on these adapted viruses. But no one appears certain how and when to end a voluntary moratorium the scientists agreed to in February, at the suggestion of the leadership of the U.S. National Institutes of Health.
"What we don't want to get into a situation of doing is saying 'I have no problem with your going ahead and ending the moratorium' and then only to find out that the next paper generates the same kind of hopefully avoidable issues that we had over the past several months," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.
Fauci said his organization is working on a new policy for local research institutions, and trying to see whether a U.S. government inter-agency committee could be tasked with setting standards of which research should and should not be done. But this could be months from completion, he admitted. Fauci said he'll talk with key flu researchers about the future of the moratorium at a meeting of a network of influenza research centres next month.
Fouchier said he wants to see the moratorium ended in a co-ordinated manner, but isn't keen to wait too long. The Dutch government has already cleared him to resume work. "If the U.S. is going to take a year from now, I don't think it's wise that the whole world is waiting for what the U.S. will be doing."