Older people who have been infected with or vaccinated against seasonal flu may have a type of immunity produced by cells that protects them from the swine flu virus, U.S. researchers said on Wednesday.

They said the pandemic H1N1 virus has parts found in earlier flu strains, and some people past age 60, who may have been exposed to similar viruses in their youth, may have some latent immune cells that protect them.

"These findings indicate that human populations may have some level of existing immunity to the pandemic H1N1 influenza and may explain why the 2009 H1N1-related symptoms have been generally mild," said Carol Cardona of the University of California Davis School of Veterinary Medicine. Her study appears in the journal Emerging Infectious Diseases.

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Cardona said cell-based immunity may be serving to weaken the effects of swine flu.

"The meaning clinically is you are going to get sick but it may not be as severe if you had no immunity whatsoever," Cardona said in a telephone interview.

Cardona said much attention is given to antibodies that recognize and destroy foreign invaders.

The body also makes cells, known as cytotoxic T-cells, which secrete antiviral chemicals that kill infected cells and clear the virus from the body. It is these cells that may be offering protection.

"It's part of the primary immune response. It just is not the one that is classically measured," Cardona said.

Cardona and colleague Zheng Xing analyzed data from prior studies of the H1N1 virus, looking at short stretches of proteins known as epitopes found in regions of the virus that are less likely to change from strain to strain.

"We simply went in and reanalyzed it," Cardona said.

They found more than a dozen of these epitopes on the H1N1 virus also are found in seasonal flu viruses that have been circulating for years.

"Not every single person can process these stretches of the protein," said Cardona, which may explain why some people who get the H1N1 flu have severe illness while others have milder cases.

According to the U.S. Centers for Disease Control and Prevention, most serious cases and deaths from swine flu have been in people under the age of 65.

"It's the younger ones who are being hospitalized," the CDC's Dr. Tim Uyeki told a meeting of the Pan American Health Organization on Wednesday.

Dr. Yoshi Kawaoka of the University of Wisconsin said studies showed people born in 1918 or earlier had many antibodies against the new pandemic H1N1 and said it may more closely resemble its distant 1918 cousin.