How the Flu Virus Can Change: “Drift” and “Shift”
Influenza viruses are constantly changing. They can change in two different ways.
One way influenza viruses change is called “antigenic drift.” These are small changes (or mutations) in the genes of influenza viruses that can lead to changes in the surface proteins of the virus: HA (hemagglutinin) and NA (neuraminidase). The HA and NA surface proteins of influenza viruses are “antigens,” which means they are recognized by the immune system and are capable of triggering an immune response, including production of antibodies that can block infection. The changes associated with antigenic drift happen continually over time as the virus replicates. Most flu shots are designed to target an influenza virus’ HA surface proteins/antigens. The nasal spray flu vaccine (LAIV) targets both the HA and NA of an influenza virus.
The small changes that occur from antigenic drift usually produce viruses that are closely related to one another, which can be illustrated by their location close together on a phylogenetic tree. Influenza viruses that are closely related to each other usually have similar antigenic properties. This means that antibodies your immune system creates against one influenza virus will likely recognize and respond to antigenically similar influenza viruses (this is called “cross-protection”).
However, the small changes associated with antigenic drift can accumulate over time and result in viruses that are antigenically different (further away on the phylogenetic tree). It is also possible for a single (or small) change in a particularly important location on the HA to result in antigenic drift. When antigenic drift occurs, the body’s immune system may not recognize and prevent sickness caused by the newer influenza viruses. As a result, a person becomes susceptible to flu infection again, as antigenic drift has changed the virus enough that a person’s existing antibodies won’t recognize and neutralize the newer influenza viruses.
Antigenic drift is the main reason why people can get the flu more than one time, and it’s also a primary reason why the flu vaccine composition must be reviewed and updated each year (as needed) to keep up with evolving influenza viruses.
The other type of change is called “antigenic shift.” 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.
While influenza viruses change all the time due to antigenic drift, antigenic shift happens less frequently. Influenza pandemics occur very rarely; there have been four pandemics in the past 100 years. For more information, see pandemic flu. Type A viruses undergo both antigenic drift and shift and are the only influenza viruses known to cause pandemics, while influenza type B viruses change only by the more gradual process of antigenic drift.