The MMR vaccine is given at 18 months because that is when a child's immune system is better able to handle the vaccine. The vaccine is also given at this age because it is when a child is most likely to be exposed to the measles virus. The MMR vaccine is given in two doses, the first at 12-15 months and the second at 4-6 years. The second dose is given to ensure that the child is protected against the measles virus.