How Flu Viruses Can Change: “Drift” and “Shift”

Influenza (flu) viruses are constantly changing. They can change in two different ways.

Antigenic Drift

One way flu viruses change is called “antigenic drift.” Drift consists of  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 flu viruses replicate (i.e., infect a host and make copies of themselves). Most flu shots are designed to target the HA surface proteins/antigens of flu viruses. The nasal spray flu vaccine (LAIV) may target both the HA and NA of a flu 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. Flu viruses that are closely related to each other usually have similar antigenic properties. This means that antibodies your immune system creates against one flu virus will likely recognize and respond to antigenically similar flu 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 also is possible for a single 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 flu viruses. As a result, a person becomes susceptible to flu infection again, as antigenic drift has changed the  virus’ antigenic properties enough that a person’s existing antibodies won’t recognize and neutralize the newer flu viruses.

Antigenic drift is an important reason why people can get flu more than one time. Drift is also a primary reason why the composition of flu vaccines for use in the Northern and Southern Hemispheres is reviewed annually and updated as needed to keep up with evolving flu viruses.

Antigenic Shift

Another type of change is called “antigenic shift.” Shift is an abrupt, major change in a flu A virus, resulting in new HA and/or new HA and NA proteins in flu viruses that infect humans. Antigenic shift can result in a new flu A subtype. Shift can happen if a flu 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 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 flu viruses change all the time due to antigenic drift, antigenic shift happens less frequently. Flu pandemics occur rarely; there have been four flu 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 flu viruses known to cause pandemics, while flu type B viruses change only by the more gradual process of antigenic drift.