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': 'Measles vaccine given to contact of measles case exces protective effect within- \nSelect and explain. \nA. 1 day, B. 3 days, C. 7 days, D. 10 days', 'role': 'user'}, {'content': "Answer: C \nExplanation: - susceptible host of measles May be protected against measles with measles vaccine, provided that this is given within 3 days of exposure. - this is because , the incubation period of measles induced by vaccine is about 7 days , compared with 10 days for naturally acquired measles. Thus protective effect is obtained within 7 days. Reference : Park's textbook of preventive and social medicine, 23rd edition, pg no:149", 'role': 'assistant'}]