Earlier Flu Viruses Provided Some Immunity to Current H1N1 Influenza, Study Shows

Humans can mount two types of immune responses. One type is produced when the invading virus triggers production of protective antibodies that circulate in the bloodstream, and the other type, described above, is known as a cell-mediated immune response. It is produced when the invading virus triggers the activation of cytotoxic T-cells, a process that helps clear the virus from the body. Evidence from earlier studies suggests that cytotoxic T-cell immune immunity can be caused by either an active viral infection or by vaccination against such a virus.

The researchers note that about 80 percent of the epitopes found in seasonal influenza and flu vaccine viruses are also present in the highly pathogenic H5N1, or avian influenza, virus. They suggest that these epitopes may have protected some individuals infected with the highly pathogenic H5N1 virus through cytotoxic T-cell immunity.

However, the H5N1 virus rapidly reproduces itself and spreads so quickly within vital organs that the body may not be able to launch protective immunity, thus accounting for the high fatality rate of avian influenza.

Furthermore, only a fraction of the human population can recognize the specific epitopes necessary to cause the appropriate protective immune response, which may explain why the H1N1 2009 virus, as well as avian influenza, may vary in severity from person to person.

Xing and Cardona propose that immunity acquired from seasonal influenza or flu vaccinations may provide partial protection for patients infected with the avian influenza virus due to the shared epitopes essential for cytotoxic T-cell immunity.

This is supported by statistics from the World Health Organization indicating that there have been fewer avian influenza infections in people 40 years and older than there were in people under that age, and that the fatality rate of avian influenza was just 32 percent in the older age group but 59 percent in the younger group.

The researchers, therefore, suggest that repeated exposure to seasonal influenza viruses or flu vaccinations may have resulted in cytotoxic T-cell immunity to avian influenza, and that the same type of immunity may also have developed in people exposed to the H1N1 virus.

Funding for this study was provided by grants from the Department of Homeland Security's National Center for Foreign Animal and Zoonotic Disease Defense, and by the UC Davis Center for California Food Animal Health.