Reported Infections with Variant Influenza Viruses in the United States since 2005

When people are infected with influenza A viruses that are known to be genetically similar to viruses circulating in swine, these viruses are called “variant viruses” and denoted with a letter “v”. As of July 27, 2012, 40 cases of infection with variant influenza viruses have been reported in the United States since 2005. Of these 40 cases, 13 have been H1N1v viruses, 25 have been H3N2v viruses and 2 have been H1N2v viruses.

All 40 persons infected with variant viruses recovered from their illness. Most cases occurred in children (persons 18 or younger) and most cases reported direct or indirect exposure to swine prior to onset of illness. Limited transmission from close contact with an infected person has been observed in some investigations of human infections with variant viruses, but sustained human-to-human transmission has not been documented. Beginning in August 2011, infection with an H3N2v virus was detected in a number of people across several U.S. states. More information about H3N2v is available at Information on H3N2 Variant Influenza A Viruses.