Antigenic Drift and Shift With the Flu Virus

Influenza strains are constantly mutating. A small change to the genetic makeup of influenza strains is referred to as antigenic drift, while a major change is called antigenic shift. While these designations are mainly relevant to scientists, they help explain why you can contract the flu more than once and why the influenza vaccine is changed annually (and may be less effective in some seasons than others).

A minor change to a flu virus is known as antigenic drift. Both influenza A and B viruses undergo antigenic drift.

These mutations in the virus’s genes can lead to changes in its surface proteins hemagglutinin (HA) and neuraminidase (NA). Known as antigens, these are recognized by the immune system, prompting an immunological response that can result in illness and promote immunity in the future.

As the virus replicates, these changes in antigenic drift happen continually. Over time, these small changes accumulate and result in a new strain that is not recognized by the immune system. The antibodies created in response to having the flu in the past are unable to protect against this new version.

Antigenic drift is the reason new flu vaccines are developed every year and the reason you can get sick from the flu even though you've had it before.

Antigenic shift is a more major change in the influenza virus. This shift typically occurs when a human flu virus crosses with a flu virus that usually affects animals (such as birds or pigs).

When the viruses mutate, they shift to create a new subtype that is different from any seen in humans before.

This can happen in three ways:

• A human flu virus infects an animal, such as a pig. The same pig also gets infected by a flu virus from another animal, such as a duck. The two flu viruses mix and mutate, creating a completely new type of flu virus that can then spread to humans.
• A strain of bird flu passes to humans without undergoing any type of genetic change.
• A strain of bird flu passes to another type of animal (such as a pig) and is then passed on to humans without undergoing a genetic change.

When a major antigenic shift like this occurs, very few people have any type of immunity to the new, or "novel," flu virus.

When flu pandemics have occurred in recent history, they have been due to antigenic shift. Once such example is COVID-19, coronavirus disease 2019. Fortunately, these shifts occur only occasionally, having caused only a handful of true flu pandemics in the past century.

Antigenic drifts and shifts make it difficult to develop flu vaccines and medications that will treat influenza infections. Researchers are hoping to develop an effective vaccine that will target a part of the virus that is not affected by these changes, leading to a universal flu vaccine that will only be needed occasionally instead of every year.

Until that day comes, people will need to continue getting seasonal flu shots and taking everyday precautions to protect themselves from the flu.