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: Social & Preventive Medicine
[{'content': 'False about antigen drift- \nSelect and explain. \nA. Causes pandemic, B. Occurs due to mutation, C. Occurs more frequently, D. Affected by previous antibodies', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Antigenic shift (not drift) is responsible for pandemic.\nAntigenic drift occurs due to point mutation.\nAntigenic drift occurs more frequently (2-3 years) than antigenic shift (10-15 years).\nAntigenic drift is influenced by the presence of antibodies to the predecessor strains in the host population.', 'role': 'assistant'}]