Good question. At that age, two shots separated by 21-28 days are required for full immunity. However, you do have some immunity from the first. If your 23 month old was exposed one to four days prior to the first dose, the chances are good there will be disease. No one can say for sure what will happen with only one dose. Do your best to keep the child clean, rested and away from crowds.
Wayne Yankus, MD, FAAP
expert panelist: pediatrics