Influenza: Antigenic Drift and Antigenic Shift

Virusses mutate, and the Influenza A Virus isn't any different. But this virus (like many others) has two ways of changing, that are called Antigenic Drift and Antigenic Shift.

Antigenic drift
One way Influenza viruses change is called 'antigenic drift'. This Drift consists of small incremental changes (or mutations) in the genes of Influenza viruses that can lead to changes in the surface proteins of the virus, hemagglutinin (HA) and neuraminidase (NA). These HA and NA surface proteins of influenza viruses are 'antigens', which means they are recognized by your immune system and are capable of triggering an immune response, including production of antibodies to fight the infection.

The changes associated with antigenic drift happen continually over time as Influenza viruses replicate. These tiny genetic changes that occur in Influenza viruses over time usually produce viruses that are (still) closely related to one another. Influenza viruses that are closely related to each other usually have similar antigenic properties. This means that your will likely recognize and respond to antigenically rather similar Influenza viruses (this is called 'cross-protection').

However, these small changes in HA and NA that accumulate over time may result in viruses that are antigenically different, resulting in a loss or reduction in protection against that particular Influenza virus. When an Influenza virus has become antigenically different (or 'antigenically drifted'), this generally means that the virus' antigenic properties are different enough that the body's immune system will have problems recognizing and fighting against the virus.

Antigenic drift is an important reason why people can get Influenza multiple times over the course of their lives. Antigenic drift is also a primary reason why the composition of Influenza vaccines for use in the Northern and Southern Hemispheres is reviewed annually and updated as needed to keep up with evolving Influenza viruses.

Antigenic Shift
Another type of change is called 'antigenic shift'. Shift is an abrupt, major change in an Influenza A virus, resulting in new HA and/or NA proteins in Influenza viruses that infect humans. See here for the evolving Influenza A(H5N1) virus and here for the evolving Influenza A(H3N3) virus.

Antigenic shift can result in a new Influenza A subtype infecting people for the first time. Shift can happen if an Influenza virus from an animal population gains the ability to infect humans. Such animal-origin viruses can contain HA or HA/NA combinations that are different enough from human viruses that most people do not have immunity to the novel virus. Such a 'shift' occurred in the spring of 2009, when an H1N1 virus with genes from viruses originating 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.

While Influenza viruses evolve genetically all the time and often undergo antigenic drift, antigenic shift happens infrequently.