Normally the incubation period of measles is 10 days from exposure to appearance of first symptom (fever). But, when infection is induced artificially by injecting the live measles vaccine, the incubation period is somewhat Shortened, averaging 7 days. Why is it so ? Let’s see In natural infection, virus enters through respiratory tract and then it gains entry to blood stream. In contrast, measles vaccine is injected subcutaneously → it immediately enters into circulation. So, measles vaccine has incubation period less (3 days) than natural infection. 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.