Information on Influenza A (H3N2) Variant Viruses (“H3N2v”)

H3N2v Background

Between August and December 2011, 12 U.S. residents were found to be infected with an influenza A H3N2 variant virus (H3N2v) with the matrix (M) gene from the 2009 H1N1 pandemic virus. This M gene may confer increased transmissibility to and among humans, compared to other variant influenza viruses. This virus also has been found in swine in many U.S. states. Investigations into the human cases revealed infections with these viruses following contact with swine as well as some limited human-to-human transmission. In 2012, another case of H3N2v in a child was detected in April. Then, beginning in July 2012, multiple H3N2v human infections were identified. CDC issued a Health Advisory and held a telebriefing on this situation on August 3, 2012.
It's possible that sporadic infections and even localized outbreaks among people with this virus will continue to occur. While there is no evidence at this time that sustained human-to-human transmission is occurring, all influenza viruses have the capacity to change and it's possible that this virus may become widespread. So far, the severity of illnesses associated with this virus in people has been similar to the severity of illnesses associated with seasonal flu virus infections. Limited serologic studies indicate that adults may have some pre-existing immunity to this virus while children do not. CDC is closely monitoring human infections with all novel influenza viruses, including H3N2v viruses, and will provide more information as it becomes available.
For more general information about variant influenza viruses, see Variant (Swine Origin) Influenza Viruses in Humans.
For more detailed information about previous cases of H3N2v infection, see Reports of Human Infections with Variant Viruses.

Table. Case Count: Detected U.S. Human Infections with H3N2v by State since August 2011

States Reporting H3N2v Cases	Cases in 2011	Cases in 2012
Hawaii		1
Indiana	2	5
Iowa	3
Maine	2
Ohio		10
Pennsylvania	3
Utah		1
West Virginia	2
Total	12	17

This chart indicates the number of CDC-reported infections with H3N2v variant influenza A viruses since August 2011 and is current as of August 3, 2012. This case count will be updated weekly each Friday.
NOTE: The state totals reported by CDC may not always be consistent with those reported by state health departments. If there is a discrepancy between these two counts, data from the state health departments should be used as the most accurate number.