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': '9th month immunization given is \nSelect and explain. \nA. Measles, B. DPT, C. BCG, D. All', 'role': 'user'}, {'content': 'Answer: A \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 a significant propoion of children will contract measles in the interval between the wearing of natural protection and the introduction of the vaccine. (refer pgno:149 park 23 rd edition)', 'role': 'assistant'}]