WASHINGTON (Reuters) - People who have had repeated flu infections -- or repeated flu vaccines -- may have some protection against the new pandemic swine influenza, U.S. researchers said on Monday.

Three-year-old Charles Clarke Jr. gets an H1N1 swine flu vaccine injection in his leg at an Asthma fair at Children's Hospital in Boston, Massachusetts November 14, 2009. REUTERS/Brian Snyder

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

“What we have found is that the swine flu 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 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 likely to have severe disease, said Allison Deckhut-Augustine of the National Institute of Allergy and Infectious Diseases.

“Adults may have some pre-existing immunity for H1N1,” Deckhut-Augustine said in a telephone interview.

That does not mean older people are protected from infection, and Deckhut-Augustine stressed that people should still be vaccinated against H1N1.

Swine flu has infected millions of people globally and killed an estimated 3,900 in the United States alone, according to the U.S. Centers for Disease Control and Prevention. Drug makers are struggling to make vaccines and governments are working to vaccinate their populations.

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

“We found that the immune system’s T-cells can recognize a significant percent of the markers in swine flu,” Peters said in a statement.

DUAL PROTECTION

The human immune system has two kinds of protection. Antibody response can prevent infection, while T-cells fight infection once it has occurred.

Peters and colleagues found T-cell protection but not antibody response.

“This T-cell response decreases severity of disease but doesn’t prevent infection,” said Deckhut-Augustine, whose agency helped pay for the study and maintains the public database that Peters used.

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

“This may also suggest why children are more susceptible to severe infection and why they might need two boosts,” Deckhut-Augustine said. “They haven’t been around as long and they haven’t been exposed to different strains of H1N1 as long as adults.”

Influenza is a very mutation-prone virus and from year to year the circulating strains drift, or change slightly. This is why new vaccines must be formulated each year and why people can catch flu again and again.

The new H1N1 was a never-before-seen combination of swine flu viruses, with a sprinkling of human and avian flu virus genetic sequences. But its long-ago ancestor was an H1N1 virus first seen in the 1918 influenza pandemic that killed upwards of 50 million people.

The researchers found that the new H1N1 swine flu shared 49 percent of its epitopes with older, seasonal H1N1 strains.

Using blood from healthy donors, they found that T-cells could recognize about 17 percent of these markers.