In their study, the researchers identified a group of immunologically important sites on the 2009 H1N1 influenza virus that are also present in other influenza viruses that have been circulating for years.
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.
Zheng Xing, a project scientist, and Carol Cardona, a veterinarian and Cooperative Extension specialist, both of the UC Davis School of Veterinary Medicine, have said that these immune responses could be attributed to earlier flu exposure and vaccinations.
"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 Cardona.
"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," added Xing.
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.
The study has been published in the journal of Emerging Infectious Diseases.