According to a report by Lyn Finelli, PhD, head of Influenza Surveillance and Outbreak Response, Influenza Division, Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, from August 17 to December 23, 2011, the CDC received reports of 12 human infections with influenza A (H3N2) variant [A(H3N2)v] viruses that have the matrix (M) gene from the influenza A (H1N1)pdm09 virus.
Human infections with animal-origin influenza viruses are rare, and only 35 cases have been reported in the United States since 2005. However, the frequency with which cases have been detected has increased substantially since August 2011.
Because influenza A (H3N2)v contains a unique combination of gene segments not previously associated with human infection, little is known about age-specific population susceptibility, transmissibility, and the potential for clinical severity.
"The emergence of animal-origin influenza viruses that can be efficiently transmitted from person to person and for which there is little population immunity may increase the risk that an influenza pandemic could occur," Dr. Finelli noted.