Birds carry the H5N1 virus

Experts fear the H5N1 virus, which is lethal to humans, will mutate to allow it to spread more easily among people - and could kill up to 50 million.

They say it would take up to six months to develop and manufacture a vaccine.

But scientists from Tokyo and Wisconsin told Proceedings of the National Academy of Sciences this time could be cut with an improved genetic technique.

Reshuffling DNA

By making the manufacturing process more efficient, it should avoid scientists having to go back to the drawing board and starting again and save them a week or so of work, it is believed.

Scientists already use "reverse genetics" to make vaccines against viruses by reshuffling the DNA of the culprit virus.

The altered virus is seeded into cells, such as monkey kidney cells or chicken eggs, to generate the vaccine that does not make a person ill but prepares the human immune system to recognise and attack the wild pandemic virus.

US scientists at the University of Wisconsin-Madison, working with Japanese colleagues from the University of Tokyo, say they have tweaked this process to make it more reliable and faster.

They reduced the number of molecules, called plasmids, needed to ferry the viral genes into the monkey kidney cells.

Improved efficiency

Normally, as many as a dozen plasmids are needed to transport the DNA.

By combining certain DNA codes together they were able to reduce the number of plasmids required to three.

Lead researcher Dr Yoshihiro Kawaoka said: "By reducing the number of plasmids, we increase the efficiency of virus production."

Reducing the number of plasmids will certainly increase the efficiency Dr Wendy Barclay from Reading University Guidance: At-a-glance

This means that correct seed strains should be ready for companies to start mass producing in the lab for use to make vaccines in large quantities.

The UK has plans to purchase enough vaccine for the entire population in the event of a pandemic caused by bird flu - also known as avian flu.

Chief Medical Officer Sir Liam Donaldson said 120 million doses would be needed - two for every person in the country.

Reading University's Dr Wendy Barclay, who has been doing research into influenza vaccines, said: "The technique of reverse genetics is extremely important in making vaccines against the highly virulent strains of bird flu."

She said there were a number of candidate bird flu vaccines undergoing trials at the moment that had been made in this way.

"There are technical difficulties in performing reverse genetics. Reducing the number of plasmids will certainly increase the efficiency. It might save you a week or so if your first go using the old technique did not work.

"However, in real life, the barriers to producing these vaccines and getting them out there are not really in generating them any more.

"The barriers are in the next stage - growing them up in bulk and getting them approved and trialled. That's the big challenge now."

The UK is also stockpiling anti-viral drugs, which may help limit symptoms and reduce the chances the disease will spread, and looking at quarantine measures and arrangements for the emergency services.

Scientists are also looking at tools that could help predict how an outbreak might unfold so that governments can be best prepared.