Influenza A (H3N2)v: What Goes Around Comes Around and Around Again

Medical Author:
Medical Editor:
Medical Editor:

The original article about how influenza A (H3N2)v may have changed to become more infective in humans was first published in January 2012. However, since that time, the situation with the virus has changed again. This article update will present the newest findings on H3N2v and some insights made by the CDC. To gain a perspective about how fast situations with influenza A viruses change, the short original article will be left essentially intact and the new information will appear starting in paragraph seven.

In the early 1990s, a human strain of flu virus, influenza A H3N2, was documented to infect pigs. It's relatively rare to document this transfer of infective flu virus from humans to pigs, but it can happen when surveillance tests of human and animal infections detect the viruses and identify them. This same type of flu virus transfer can occur from pigs to humans; this occurred in the 2009 "swine flu" pandemic with the influenza A H1N1 strain of virus. Usually there is some major or at least minor change or modification of the flu virus genome that allows the flu virus to more easily adapt to infecting, replicating, and reinfecting a new host population. This flu season that officially began in October 2011 has a new flu type detected in it; the virus is closely related to that influenza A H3N2 virus that infected pigs in the 1990s, and this time the evidence suggests the infected pigs transferred the virus back to humans with some modifications. The modified virus is termed influenza A (H3N2)v.