If you were vaccinated between 1968 and 1989, you likely received just one dose of the measles vaccine, instead of the two doses that are standard today. One dose alone is highly effective and for most people, it provides more than enough protection, says Dr. Adam Ratner, a pediatric infectious disease specialist in New York City and author of Booster Shots, a book on the history of measles.

But Ratner says there are several situations in which the CDC recommends an additional dose of measles vaccine for adults who are considered at high risk. That includes people who are in college settings, work in health care, live or are in close contact with immuno-compromised people, or are traveling internationally.