Seniors' H1N1 immunity could come from seasonal flu exposure, vaccinations

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Seniors' apparent immunity from swine flu could indicate that the human population, as a whole, has a preexisting immunity to H1N1. Such thinking would help explain why H1N1 symptoms have been generally mild, researchers said Wednesday.

Scientists at the University of California-Davis said certain elements in the H1N1 strain of influenza have been found in past seasonal strains of the flu. Proteins in viruses called epitopes are less likely to change from strain to strain. When they reviewed the information on swine flu, researchers found the H1N1 strain contains roughly a dozen such epitopes that appeared in recent seasonal strains of the flu. Some individuals over the age of 60 were likely exposed to those strains in their youth, researchers say.

Researchers also note that, while most immunity measures focus on anti-bodies, that's not the body's only defense against viruses. The body also makes cytotoxic T-cells, which produce antiviral chemicals. It is this cellular protection that could also be contributing to the more mild symptoms associated with the disease, according to researchers. The study appears in the journal Emerging Infectious Diseases.