We are told the pandemic will most likely begin when a bird flu virus, like H5N1, enters a human body and comes into contact, and shares genetic information, with a virulent strain of human flu.
New research indicates the two viruses will fit together very well, but the research also raises doubts on the likelihood of a bird flu virus ever becoming so virulent amongst humans as to cause a pandemic :
An experiment mating H5N1 avian flu viruses and a strain of human flu in a laboratory produced a surprising number of hybrid viruses that were biologically fit, a new study reveals.Is the general thinking wrong then on the potential likelihood of H5N1 becoming pandemic, and killing hundreds of thousands, if not millions, of people? We are told, most often by World Health Organisation spokespeople, that a bird flu pandemic amongst humans is not a case of if, but when. But what if it's a case of "not likely"?
And while none of the offspring viruses was as virulent as the original H5N1, about one in five were lethal to mice at low doses, showing they retained at least a portion of the power of their dangerous parent.
The work suggests that under the right circumstances - and no one is clear what all of those are - the two types of flu viruses could swap genes in a way that might allow the H5N1 virus to acquire the capacity to trigger a pandemic. That process is called reassortment.
"This study is just showing exactly that: There is a risk this virus can successfully reassort with a human virus," said Richard Webby, director of the World Health Organization's collaborating centre for influenza research at St. Jude Hospital in Memphis, Tenn.
"The problem is we don't know at this stage whether there's a benefit to these H5N1 viruses in doing that."
Nor can anyone say why, if the viruses swapped genes so readily in the laboratory, that hasn't seemed to have happened in the parts of the world where H5N1 has been circulating for years.Reassortment is one of two ways in which a pandemic virus can evolve. The other is for a bird virus to acquire a number of mutations that allow it to more easily infect people and transmit among them.
The latter, called adaptive mutation, is thought to be the way the 1918 Spanish flu virus emerged. The viruses responsible for the milder pandemics of 1957 and 1968 arose through the mixing of human and avian flu virus genes.
This work, done at the CDC, was conducted to study the reassortment potential of H5N1 and H3N2 viruses. H3N2 is one of two human influenza A viruses that cause disease during flu season.Reassortment studies can be done one of two ways. One involves simultaneously infecting cells with the two viruses and seeing what nature produces. The other involves making viruses by piecing together combinations of synthesized human and avian genes.
"It's like Lego," Donis, head of the molecular virology and vaccines branch, says of this approach, which was the one used for this study.
But this is a game of Lego where it's not clear from looking at the pieces which will go together into a structure that will hold. "We really don't understand the rules of engagement for playing the Legos. We don't know what makes these things connect well or not connect well," he admits.