This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Epidemiology, Subject name: Social & Preventive Medicine
[{'content': 'Vaccine not given between 0-6 months is- \nSelect and explain. \nA. HIV, B. Measles, C. BCG, D. Tetanus', 'role': 'user'}, {'content': 'Answer: B \nExplanation: .the principal problem of measles immunization is timing,immunization before the age of 9 months runs the risk of the vaccine being rendered ineffective by the natural antibodies acquired through the mother.immunization later than 9 months means that a significant propoion of children will contract measles in the interval between wearing off natural protection,and the introduction of the vaccine. ref:park&;s textbook,22 nd ed,pg 140', 'role': 'assistant'}]