Antigenic Shift and the H1N1 Influenza A Virus 

Antigenic Shift and the H1N1 Influenza A Virus
This animation provides an overview of the principles of antigenic shift and the H1N1 influenza A virus. One way influenza viruses change is called “antigenic shift” (the other is “antigenic drift”.) Antigenic shift is an abrupt, major change in an influenza A virus, resulting in new HA and/or new HA and NA proteins in influenza viruses that infect humans. Shift can result in a new influenza A subtype in humans. One way shift can happen is when an influenza virus from an animal population gains the ability to infect humans. Such animal-origin viruses can contain an HA or HA/NA combination that is so different from the same subtype in humans that most people do not have immunity to the new (e.g., novel) virus. Such a “shift” occurred in the spring of 2009, when an H1N1 virus with genes from North American Swine, Eurasian Swine, humans and birds emerged to infect people and quickly spread, causing a pandemic. When shift happens, most people have little or no immunity against the new virus.