Antigenic Drift and Shift With the Flu Virus

Influenza strains are constantly mutating. Small changes to the genetic makeup of influenza strains are referred to as antigenic drift, while a major change is called antigenic shift.

When the flu strain mutates, our immune system recognizes it as a new virus. The antibodies created in response to having the flu in the past are unable to protect against the new strain.

These genetic mutations are why we can contract the flu more than once, and the reason the influenza vaccine is changed annually and may be less effective in some flu seasons than others.

A minor change to the 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). These proteins, known as antigens, 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.

This antigenic drift is the reason we need new flu vaccines every year and the reason we can get sick from the flu multiple times in our lives.

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:

1. 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 can mix and mutate, creating a completely new type of flu virus that can then spread to humans.
2. A strain of bird flu passes to humans without undergoing any type of genetic change.
3. 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 known history, it has been due to an antigenic shift in the virus. Fortunately, these shifts occur only occasionally, having caused only four true flu pandemics in the past century.

This major antigenic shift only happens to influenza A viruses, not in influenza B viruses (they only undergo antigenic drift).

These antigenic drifts and shifts make it difficult to develop flu vaccines and medications that will treat it. 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, we will need to continue getting seasonal flu shots and taking everyday precautions to protect ourselves from the flu.