Human populations may have some level of existing immunity to the pandemic H1N1 influenza, say researchers

University of California, Davis, researchers studying the 2009 H1N1 influenza virus, formerly referred to as "swine flu," have identified a group of immunologically important sites on the virus that are also present in seasonal flu viruses that have been circulating for years. These molecular sites appear to result in some level of immunity to the new virus in people who were exposed to the earlier influenza viruses.

More than a dozen structural sites, or epitopes, in the virus may explain why many people over the age of 60, who were likely exposed to similar viruses earlier in life, carry antibodies or other type of immunity against the new virus, immune responses that could be attributed to earlier flu exposure and vaccinations.

Researchers Zheng Xing, a project scientist, and Carol Cardona, a veterinarian and Cooperative Extension specialist, both of the UC Davis School of Veterinary Medicine, report their findings online in the journal of Emerging Infectious Diseases. The report will appear in the November print edition of the journal, published by the Centers for Disease Control and Prevention.

"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," Cardona said.

"Our hypothesis, based on the application of data collected by other researchers, suggests that cell-mediated immunity, as opposed to antibody-mediated immunity, may play a key role in lowering the disease-causing ability, or pathogenicity, of the 2009 H1N1 influenza," Xing added.

He noted that immune responses based on production of specific cells, known as cytotoxic T-cells, have been largely neglected in evaluating the efficacy of flu vaccinations. In this type of immune response, the T-cells and the antiviral chemicals that they secrete attack the invading viruses.