Homeland Security

Antigenic Drift vs. Antigenic Shift


Antigenic Drift

Antigenic 'drift' occurs in HA and NA, and is associated with seasonal epidemics.

Each year's flu vaccine contains three flu strains -- two A strains and one B strain -- that can change from year to year.

  1. After vaccination, your body produces infection-fighting antibodies against the three flu strains in the vaccine.
  2. If you are exposed to any of the three flu strains during the flu season, the antibodies will latch onto the virus's HA antigens, preventing the flu virus from attaching to healthy cells and infecting them.
  3. Influenza virus genes, made of RNA, are more prone to mutations than genes made of DNA.
  4. If the HA gene changes, so can the antigen that it encodes, causing it to change shape

If the HA antigen changes shape, antibodies that normally would match up to it no longer can, allowing the newly mutated virus to infect the body's cells. This type of genetic mutation is called "antigenic drift."

Influenza viruses can change through antigenic drift, which is a process in which mutations to the virus genome produce changes in the viral H or N. Drift is a continuous ongoing process that results in the emergence of new strain variants. The amount of change can be subtle or dramatic, but eventually one of the new variant strains becomes dominant, usually for a few years, until a new variant emerges and replaces it. In essence, drift affects the influenza viruses that are already in worldwide circulation. This process allows influenza viruses to change and re-infect people repeatedly through their lifetime and is the reason the influenza virus strains in vaccine must be updated each year.


Antigenic Shift

Antigenic 'shift' occurs in HA and NA and is associated with pandemics.

The genetic change that enables a flu strain to jump from one animal species to another, including humans, is called antigenic shift. Antigenic shift can happen in three ways:

Antigenic Shift 1

  • A duck or other aquatic bird passes a bird strain of influenza A to an intermediate host such as a chicken or pig.
  • A person passes a human strain of influenza A to the same chicken or pig.
  • When the viruses infect the same cell, the genes from the bird strain mix with genes from the human strain to yield a new strain.
  • The new strain can spread from the intermediate host to humans.

Antigenic Shift 2

  • Without undergoing genetic change, a bird strain of influenza A can jump directly from a duck or other aquatic bird to humans.

Antigenic Shift 3

  • Without undergoing genetic change, a bird strain of influenza A can jump directly from a duck or other aquatic bird to an intermediate animal host and then to humans.

The new strain may further evolve to spread from person to person. If so, a flu pandemic could arise.

In contrast to drift, pandemic viruses arise through a process known as antigenic shift. In this process, the surface existing viral H and N proteins are not modified, but are replaced by significantly different H and Ns. Since influenza A viruses that bear new (or novel) H or H/N combinations are perceived by immune systems as new, most people do not have pre-existing antibody protection to these novel viruses. This is one of the reasons that pandemic viruses can have such severe impact on the health of populations.

Download the high resolution version of the Antigenic Shift illustration.



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