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However, because the virus keeps evolving, the jabs can never offer full protection.

Scientists said the findings could be used as the basis for a new universal vaccine, which would stimulate the production of the crucial cells, to boost the immune systems against all strains of the virus.

Prof Ajit Lalvani from the National Heart and Lung Institute, who led the study, said: "New strains of flu are continuously emerging, some of which are deadly, and so the Holy Grail is to create a universal vaccine that would be effective against all strains of flu."

Scientists at Imperial College London used the pandemic in 2009 as a "unique" natural experiment to work out why some people went down with a nasty bout of the flu while others didn't.

Hundreds of staff and students donated their blood samples just as the pandemic took off.

Over the next two flu seasons they reported back on how their health fared and whether they came down with flu.

Volunteers gave blood samples and nasal swabs and every three weeks filled in a survey about their health.

If they came down with flu symptoms they took a nasal swab and sent it back to the lab.

Those who fell badly ill with the flu had fewer of the CD8 T cells, while those who caught flu but had no symptoms or escaped the virus altogether had more of these cells.

Prof Lalvani said: "The 2009 pandemic provided a unique natural experiment to test whether T cells could recognise, and protect us against, new strains that we haven't encountered before and to which we lack antibodies."

"Our findings suggest that by making the body produce more of this specific type of CD8 T cell, you can protect people against symptomatic illness. This provides the blueprint for developing a universal flu vaccine."

Scientists already know how to develop a vaccine which would stimulate the production of more of the T cells, researchers said.

"Now that we know these T cells may protect, we can design a vaccine to prevent people getting symptoms and transmitting infection to others. This could curb seasonal flu annually and protect people against future pandemics," Prof Lalvani said.

The cells have been known to play a part in protecting people from disease, with more activity among them among HIV sufferers who remained well.