Epidemiologic implications of changes in the influenza virus genome. Antigenic variation Influenza viruses are able to change their antigenicity and thereby evade their host's immune mechanisms. This is why major epidemics of influenza occur repeatedly and why new vaccines have to be devised and produced very frequently if they are to provide effective protection. Figure 6 shows the virus nucleocapsid of the influenza A virus.
Antigenic variation is a frequent event with type A influenza viruses, often occurring every year.
It occurs less frequently with type B viruses and has not been associated with type C viruses. Two kinds of antigenic variation occur. A and B viruses. They involve minor changes to the structure of the HA and NA spikes. As a result of random mutations in the genes which code for HA or less commonly NA, there are some small alterations in the structure of the spikes.