This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: general microbiology, Subject name: Microbiology
Secular trend in influenza is due to - 
Select and explain. 
A. Antigenic shift, B. Antigenic drift, C. Endemicity, D. Virulence
A: Antigenic shift
Answer: A: Antigenic shift 
Explanation: Antigenic shift,is an brupt drastic discontinuation variation in the antigenic structure resulting in a novel virus strain unrelated antigenically to predecessor strains. Such chamges may involve hemagglutinin , neuraminidase or both. Antibodies to predecessor viruses do not neutralise the new variants and can therefore spreads widely in the population causing major epidemics or pandemics. The changes involved in antigenic shifts are too extensive to be accounted for by mutation REF:Ananthanarayan & Panicker's Textbook of Microbiology 8th edition pg no:496