Genetic Re-assortment of Influenza A Viruses

Immunity against Influenza viruses

1. A host is infected with a particular influenza virus strain.
2. Antibodies are formed against that strain.
3. The antibodies formed protect the host against re-infection by the same virus strain.

The Emergence of “New Viruses” through Antigenic Shift and Drift

On the influenza virus, the surface proteins haemagglutinin and neuraminidase are important in inducing the formation of antibodies.

Changes in the surface proteins result in antigenic changes in the virus. The immune system no longer recognizes the virus strain and the host is susceptible to re-infection.

Influenza viruses can change in two different ways.

1. Antigenic drift
   • Gradual changes in an already circulating virus
   • Eventually the virus changes enough so that most of the population is susceptible to re-infection. There is then an influenza epidemic.
   • Size and severity of the epidemic is dependent on the degree to which the virus is different from those already experienced by the population.
   • Influenza viruses are constantly changing by antigenic drift
   • Type A and Type B influenza viruses undergo changes by antigenic drift.
   • The WHO Global Influenza Programme monitors antigenic changes and the adjustments required in influenza vaccines
     
     
2. Antigenic shift
   • The sudden emergence of new antigenically different influenza A sub-type.
   • The host population has no immunity against the new sub-type and an influenza pandemic follows.
   • Antigenic shift only occurs occasionally
   • Only Type A Influenza viruses change by antigenic shift
   • Antigenic shift is of public health concern as subtypes from different species may then be able to infect humans.

Mechanisms of Antigenic shift

Antigenic shift in man probably occurs through one of these mechanisms:

1. Direct transfer of a virus from another species.
   
   
2. Genetic re-assortment of avian and human influenza viruses infecting the same host. 
   • Believed to take place in pigs. See swine influenza.
   • New evidence indicates that it may also take place in humans.
   • The new virus that emerges may be an avian virus containing enough human influenza genes to allow human to human transmission.
     
     
3. Re-emergence of a virus that may have caused an epidemic years earlier.

For more information see:
The Center for Disease control and prevention, Influenza information.