The other 2 arteries that bring blood to the brain, called the vertebral arteries, supply the brainstem. Plaque in the vertebral arteries is much less likely to break off and cause a stroke, but narrowing in the vertebral arteries can cause problems if the brainstem does not get enough blood flow.
A child should receive the first shot when he is between 12-15 months, and the second when he's between 4-6 years of age. If you're not sure if you have had the diseases or the vaccines (prior to 1971 it was given in three separate shots), you can get the MMR vaccine as an adult.
All infants younger than 24 months should receive four doses of the vaccine, the first one at 2 months. The next two shots should be given at 4 months and 6 months, with a final booster that should be given at 12 to 15 months. Children who do not get their shot at these times should still get the vaccine.