This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: miscellaneous, Subject name: Microbiology
Which vaccine strain is changed every yearly 
Select and explain. 
A. Influenza, B. Rabies, C. Ebola, D. Hepatitis
A: Influenza
Answer: A: Influenza 
Explanation: * 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