Antigenic Drift vs. Shift: How Flu Viruses Change

Understanding how the flu virus changes over time

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In the influenza virus, antigenic drift happens when proteins called antigens undergo small changes over time. Antigenic shift is when an abrupt change results in a completely different antigen.

Antigenic drift and shift 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.

This article discusses antigenic shift vs. drift and what this means for the annual flu season and the flu vaccine.

boy with flu

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Antigenic Drift

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 promote immunity in the future.

As the virus replicates, these changes happen continually. Over time, the small changes accumulate and result in a new strain that is not recognized by the immune system. This means 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

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.

Examples of antigenic shift include:

  • 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.

Antigenic drift and shift can also occur in other virus types such as coronaviruses.

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

Major antigenic shift only happens to influenza A viruses, the type that causes the most severe human illness. Influenza B viruses, which tend to cause less severe illness, only undergo antigenic drift.

Drifts, Shifts, and the Flu Vaccine

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 then, people will need to continue getting seasonal flu shots and taking everyday precautions to protect themselves from the flu.

Summary

Antigenic drift and shift are genetic changes that happen to the influenza virus. Antigenic drift describes small changes to antigens that occur over time, while antigenic shift describes major changes that happen abruptly. These two processes are the reason why it is possible to get the flu more than once and why vaccines need to be updated every year.

Researchers are working on a universal flu vaccine that targets parts of the flu virus that aren't subject to antigenic drift and shift. Until such a vaccine is created, an annual flu shot is still the best way to protect yourself from influenza.

2 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. U.S. Centers for Disease Control and Prevention. How flu viruses can change: "Drift" and "shift".

  2. National Institute of Allergy and Infectious Diseases. Universal influenza vaccine research.

By Kristina Herndon, RN
Kristina Herndon, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.