PEOPLE who have had repeated flu infections - or repeated flu vaccines - may have some protection against the new Influenza A (H1N1) pandemic, United States researchers reported.

They found evidence that the human immune system can recognise bits of the H1N1 virus that are similar to older, distantly-related H1N1 strains.

"What we have found is that (the new virus) has similarities to the seasonal flu, which appear to provide some level of pre-existing immunity.

"This suggests that it could make the disease less severe in the general population than originally feared," said Dr Alessandro Sette, director of the Center for Infectious Disease at California's La Jolla Institute.

The study, published in the Proceedings Of The National Academy Of Sciences, may also help explain why many older people are less severely affected by the new virus.

Said Dr Allison Deckhut-Augustine of the National Institute of Allergy and Infectious Diseases: "Adults may have some pre-existing immunity."

That does not mean older people are protected from infection, and Dr Deckhut-Augustine said vaccinations should not be avoided on account of the new research results.

H1N1 has infected millions of people globally and killed an estimated 3,900 in the US alone.

Dr Bjoern Peters and colleagues at the La Jolla Institute looked at flu epitopes - molecular markers or structures that the immune system recognises - dating back 20 years.

"We found that the immune system's T-cells can recognise a significant per cent of the markers in swine flu," Dr Peters said.

The immune system has two kinds of protection. Antibody response can prevent infection while T-cells fight infection once it occurs. Dr Peters' team found T-cell protection but not antibody response.

"This T-cell response decreases the severity of disease but doesn't prevent infection," said Dr Deckhut- Augustine, whose agency helped pay for the study.

The effect could be cumulative, Dr Peters said, which could explain why people over 50 seem less likely to get noticeable H1N1 infections.

Said Dr Deckhut- Augustine: "This may also suggest why children are more susceptible to severe infection."