Amid a measles outbreak in the U.S. that's the worst in 25 years, the Centers for Disease Control and Prevention is saying that some adults who were vaccinated against measles decades ago may need a new dose. Up to 10 percent of the 695 cases that have been confirmed were in people who had one or two doses of the vaccine. The CDC recommended that people who are living in or traveling to outbreak areas should check their vaccination status and consider getting a new dose. People vaccinated in the U.S. since 1989 will most likely have gotten two doses of the measles, mumps and rubella (MMR) shot, which is considered the standard for protection. But anyone vaccinated between 1963 and 1989 would likely have received only one dose, and many of those in the earlier years in that group got an inactivated version of the virus, both of which are less effective. Americans born before 1957 are considered immune, since they would have been exposed to the virus in an outbreak. If immunization records can't be found, a blood test can show if someone has immunity.