Which vaccine strain is changed every yearly

* Influenza virus has two impoant antigens: Haemagglutinin (H) and Neuraminidase (N) * These antigens undergo periodic antigenic variations * Major antigens variations are seen only with influenza A viruses and may be associated with pandemics - called as antigenic shifts. * Minor variations causing outbreaks are called as antigenic drifts. * Because of these variations, vaccines should be modified according to the current prevalent strain. Ref:- Harrisons T.B of internal medicine 19th edition; pg num:- 1209