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Influenza A (H3N2) Variant Virus

Influenza viruses that normally circulate in pigs are called “variant” viruses when they are found in people. Influenza A H3N2 variant viruses (also known as “H3N2v” viruses) with the matrix (M) gene from the 2009 H1N1 pandemic virus were first detected in people in July 2011. The viruses were first identified in U.S. pigs in 2010. In 2011, 12 cases of H3N2v infection were detected in the United States (Indiana, Iowa, Maine, Pennsylvania, and West Virginia). In 2012, 309 cases of H3N2v infection across 12 states were detected. In 2013, 19 cases of H3N2v across five states were detected. These infections have mostly been associated with prolonged exposure to pigs at agricultural fairs. Limited human-to-human spread of this virus has been detected in the past as well (Iowa, West Virginia) but no sustained or community spread of H3N2v has been identified at this time. It's possible that sporadic infections and even localized outbreaks among people with this virus will continue to occur. The Centers for Disease Control and Prevention (CDC) continues to monitor this situation closely and will report cases of H3N2v and other variant influenza viruses weekly in FluView and on the case count tables on this website. CDC has developed guidance for the public to protect against H3N2v, and guidance for public health and health care workers.

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