This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Communicable diseases, Subject name: Social & Preventive Medicine
[{'content': 'Duration of immunity after typhoid vaccination is for - \nSelect and explain. \nA. 3 years, B. 5 years, C. 7 years, D. 6 months', 'role': 'user'}, {'content': 'Answer: A \nExplanation: <p>Vi polysaccharide vaccine:- Composed of purified Vi capsular polysaccharide from Ty2 S. Administration- subcutaneous/ intramuscularly. Age group more than 2 years. Only one dose required. Confers protection 7 days after injection. Efficacy-85% Duration of immunity-3yrs. To maintain protection,revaccination recommended every 3 years. {Reference: park&;s textbook of preventive and social medicine, 23rd edition, pg no.237}</p>', 'role': 'assistant'}]