Earlier flu viruses provided some immunity to current H1N1 influenza, study shows

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.

About 2009 H1N1 influenza

The 2009 H1N1 virus is a new strain of influenza that first appeared in the United States in April 2009. Early on, it was referred to as "swine flu" because it was genetically similar to influenza viruses that normally occur in pigs in North America. Further study, however, revealed that the virus actually included genes from viruses found in birds and humans, as well as pigs.

At first, this H1N1 influenza virus apparently caused a high number of deaths among patients in Mexico and among people with certain pre-existing medical conditions. But as it has progressed to become a pandemic or geographically widespread virus, H1N1 has caused relatively mild symptoms and few deaths.

One hallmark of this new influenza virus, according to the Centers for Disease Control and Prevention, has been the presence of pre-existing antibodies against the virus in about one third of H1N1 2009 patients over the age of 60, a phenomenon that suggested some levels of immunity may have existed to the new pandemic H1N1 virus. The UC Davis research