Scientists have used genetic modification in an early step towards creating a pandemic flu vaccine.
Researchers are looking for the best and easiest to produce vaccine
The US Centers for Disease Control created the vaccine by putting a gene from a strain of the deadly H5N1 type of bird flu into a cold virus.
This was then developed in cell cultures, rather than hens' eggs used in conventional vaccine development.
Experts said the Lancet paper was technically interesting, but not immediately useful.
The growing incidence of cases of H5N1 in humans is hastening scientists' search for a vaccine which would be effective in a flu pandemic.
The problem is that it would not be the existing strain of the virus being seen in birds which would cause that pandemic.
The fear is that H5N1 will mutate or combine with a human flu, and spread quickly and easily.
The traditional way to make a vaccine is to inject a slightly altered virus - which will not cause disease - into fertilised hens' eggs.
But it takes around six months for the whole process to be completed, and scientists are looking at ways to speed this up.
The team from the CDC took one gene from a version of H5N1 seen in Hong Kong in 1997.
They put this haemugglutinin gene into a common cold virus and used human kidney cells to grow the vaccine.
They then vaccinated mice, who were shown to be protected against other versions of H5N1 seen in 2003.
And scientists predict using GM would speed up the production of vaccines, although no-one is yet sure by how much.
'Very early days'
Dr Suryaprakash Sambhara, of the CDC, and Dr Suresh Mittal, of Purdue University in Indiana, said: "This approach is a feasible vaccine strategy against existing and newly emerging viruses of highly pathogenic avian influenza to prepare for a potential pandemic."
But Professor John Oxford, a virology expert at Barts and the Royal London School of Medicine and Dentistry, said genetic modification was something a number of flu vaccine researchers were looking into.
And he said: "It could be of use if a pandemic hits in three of four years, but not before then."
Dr John Wood, a flu specialist at the National Institute for Biological Standards and Control, said he believed this was the first published data on creating a pandemic vaccine using genetic modification.
But he added: "It is very early days. This vaccine has to jump over a lot of hurdles before it could be used."