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
Typhoid revaccination is recommended very ___ years in endemic area - 
Select and explain. 
A. 1, B. 3, C. 5, D. 10
B: 3
Answer: B: 3 
Explanation: <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>