A way of creating more effective vaccines which could protect against a broad range of flu viruses has been reported by US researchers.

A different seasonal flu jab is produced every year as the virus is a constantly shifting target.

This animal study, published in the journal Nature, showed a single jab could protect against multiple strains.

Flu scientists said it was an important advance, but a vaccine which could defeat all flu was a long way off.

While there are different strains of flu circulating each year, there are bits of the flu virus which do not change.

Many groups of researchers believe that targeting these weak spots could lead to a single, universal flu vaccine.

Synthetic biology

The normal seasonal flu jab is made by growing the virus in chicken eggs. It is then inactivated and injected into people to train the immune system to fight off that virus.

A group at the pharmaceutical company Sanofi used a different approach to design a new protein which was half virus.

Spikes which stick out from the surface of the virus, which hardly vary between strains, were fused with a 'transporter protein' which is naturally found in blood.

Groups of these hybrid proteins then spontaneously formed tiny spheres, which were tested in ferrets.

Flu researchers use ferrets as they are can be infected with human viruses, which results in similar symptoms.

The vaccine gave the animals immunity against multiple batches of flu ranging from viruses circulating in 1934 through to 2007.

Dr Gary Nabel, the chief scientific officer at Sanofi, told the BBC: "We think this is a step down the path towards a universal vaccine. It's not a universal vaccine yet.

"There's lots of research in the early phases and this looks as good as anything out there."

Not everything

The spike used in the vaccine was haemagglutinin, but there are many different types of haemagglutinin. It is how viruses are classified - swine flu in 2009 was H1N1, with the H standing for haemagglutinin.

This vaccine was designed to protect against H1 flu viruses. It would not protect against others such as the current bird flu in China, H7N9.

Prof Sarah Gilbert, who works on universal vaccines at Oxford University, told BBC News: "It is an improvement on the current vaccine. It's not a 'universal vaccine' but it's definitely a step in the right direction."

She said it might be able to get over the problems of "mis-match" when there are differences between the seasonal vaccine and the flu being targeted.

However, the vaccine has not yet been tested in people. Clinical grade vaccine has not yet been developed so even safety trials are thought to be a year away.

There is a risk that the flu virus could find ways to evade the vaccine.

Prof Wendy Barclay, from Imperial College London, said: "I think the important question to explore in the field now is...will the virus be able to escape by 'drift' like it does each year to our natural antibody response, or can it be 'pinned in' by the immune response induced by this new era of vaccines?"

Dr Nabel agreed that viruses could be difficult to pin down: "It is like squeezing a balloon. You squish one place and another pops out. The viruses are very clever and under pressure they find a new way to escape."