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A universal vaccine that protects against all strains of the flu virus may be a step closer.

The annual flu vaccine currently protects against four strains of the virus which scientists believe are most likely to be in circulation in the coming winter.

But a vaccine that protects us from all types of flu — which kills between 10,000 and 30,000 people a year in the UK — is one of the holy grails of medicine.

There are three different types of flu virus affecting humans — A, B and C, with various sub-types (including 29 of type A alone) which may mutate over time.

Scientists look at patterns of flu in other parts of the world to help decide which strains to include in the annual flu vaccine given here — but that has to be months in advance, to allow time for the vaccine to be made.

The vaccine will include two A strains and two B strains thought most likely to be circulating in the coming winter (type C strains are not included, as they usually cause only mild symptoms).

But scientists don't always get the selection right. In 2014/15, for instance, the flu jab ended up being only 3 per cent effective at preventing cases of flu because the strains contained in the vaccine were so out-of-kilter with what was circulating.

In 2015, almost 30,000 people in England and Wales alone died of flu and pneumonia, compared with 25,000 the year before.

Now, by adopting a new approach, scientists at the U.S. National Institute of Allergy and Infectious Diseases have made a 'major step forward' in the search for a universal flu vaccine. Current vaccines target the 'head' of proteins called hemagglutinins, which are found on the surface of the flu virus. This 'head' varies between strains and changes over time — which is one reason why the vaccine must be updated each year.

The new research focuses on the 'stem' of the protein. Because this is sufficiently similar between different strains, the theory is this will offer protection against all strains.

To ensure the immune system focuses on the stem and not the head (as it has been programmed to do by previous flu vaccines), scientists lopped off the head and replaced it with an engineered one which the body's immune system would not recognise.

'The only thing the immune system can see is the stem,' said Sarah Andrews, an immunologist and one of the researchers.

To test this new approach, which involved just the influenza A strain, 52 volunteers were injected with two doses of the experimental vaccine. The hope was that this would offer protection against all A strains of the virus.

Results showed the vaccine appeared to be safe; around a fifth of recipients had soreness at the injection site or a headache, in line with the side-effects reported with the standard flu shot.

One year after being vaccinated, participants were still showing antibodies against all type A flu strains, reported the journal Science Translational Medicine last month.