This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
Measles vaccine given to contact of measles case exes protective effect within - 
Select and explain. 
A. 1 day, B. 3 days, C. 7 days, D. 10 days
C: 7 days
Answer: C: 7 days 
Explanation: Ans. is 'c' i.e., 7 days o Normally the incubation period of measles is 10 days from exposure to appearance of first symptom (fever). o But, when infection is induced aificially by injecting the live measles vaccine, the incubation period is somewhat shoened, averaging 7 days. Why is it so ? Lets see o In natural infection, virus enters through respiratory tract and then it gains entry to blood stream. o In contrast, measles vaccine is injected subcutaneously it immediatly enters into circulation. So, measles vaccine has incubation period less (3 days) than natural infection. o For this reason, measles vaccine should be given within 2-3 days after exposure, so that vaccine virus can induce immunity before the natural virus can cause clinical illness.