This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
False about antigen drift ? 
Select and explain. 
A. Causes pandemic, B. Occurs due to mutation, C. Occurs more frequently, D. Affected by previous antibodies
A: Causes pandemic
Answer: A: Causes pandemic 
Explanation: Ans. is 'a' i.e., Causes pandemic Antigenic shift (not drift) is responsible for pandemic. Antigenic drift occurs due to point mutation. Antigenic drift occurs more frequently (2-3 years) than antigenic shift (10-15 years). Antigenic drift is influenced by the presence of antibodies to the predecessor strains in the host population.