<prompt> Which vaccine strain is changed every yearlyInfluenzaRabiesEbolaHepatitis  <response>: 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 