The structure of the influenza virus does not remain the same over the time. It changes in two methods, antigenic drift and antigenic shift. All types of Influenza viruses change constantly by antigenic drift whereas antigenic shift only occurs once in a while. On top of this, Influenza Type A viruses are more susceptible to experience both methods of changes but Influenza Type B viruses can change only by antigenic drift.

a) Antigenic Drift

Antigenic drift is the minor mutation of the surface glycoproteins, namely hemagglutinin (HA) and neuraminidase (NA) of the influenza virus. Antigenic drift occurs over a long period of time and it is a gradual process.

This is the process in which antigenic drift occurs. Firstly, when a human has been infected by the influenza virus, he/she develops an antibody against the particular strain of virus after he/she recovers. Hence, in order for the influenza virus to continue being able to infect humans, it changes its outer surface glycoproteins which are the hemagglutinin (HA) and neuraminidase (NA) by a slight mutation. These minor changes in the outer surface of the influenza virus may deceive antibodies to allow themselves to infect the body.

Hence, due to the process of antigenic drift, influenza viruses are able to infect the same person for multiple times. Another result caused by antigenic drift is the seasonal epidemics that occur in winter every year. In order to prevent these occasional epidemics, scientists have to create vaccines based on regular global surveillance around the world. It is also for safety precautions that people get a flu shot every now and then to protect themselves.

b) Antigenic Shift

In contrast to antigenic drift, antigenic shift is a major change to the virus structure to create an absolutely new subtype of influenza virus.

Antigenic shift can occur in two ways. The first method is by direct human contact with poultry infected with avian influenza virus and the second method is by the mixing of the human influenza virus with the avian influenza virus by a process named genetic reassortment . A new subtype which humans have little resistance and immunity to will develop and a possible pandemic may occur.

This process of antigenic shift in influenza viruses has been hastened by species which are capable of being infected by different types of the influenza virus. This species, in particular, is the pigs. The pigs are capable of being infected by the avian influenza virus, the human influenza virus and also the swine influenza virus. Hence, there is a very high possibility for genetic reassortment to occur in a pig infected by different types of the virus which will result in a novel influenza virus.

The results of genetic reassortment in pigs are undesirable. A new human influenza virus with the outer surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA) of the avian influenza virus may be created. If this is the case, human to human transmission of the influenza virus will be possible because the human immunity system is unable to detect the new glycoproteins on the influenza virus. As a result, a global pandemic may happen.

Although direct contact with poultry infected with avian influenza virus does not usually lead to influenza infections in humans, there were cases in which humans have been infected by certain subtypes of the avian influenza virus.

Below are some of the conditions required for a global influenza pandemic to occur:

? A novel influenza virus which humans have no protection against is introduced

? A novel influenza virus which is able to cause serious illness

? A novel influenza virus which is able to be transmitted from human to human easily