evidence: Some adults who received the measles vaccine between 1963 and 1967 may not be protected from the virus, according to the Centers for Disease Control and Prevention (CDC). That's because when the measles vaccine first became available, in 1963, there were two versions and only one was effective. [...] This year is on track to be the worst year for measles outbreaks in 27 years, according to CDC data. From January 1 to April 11, there were 555 individual confirmed cases of measles in 20 states. [...] The upshot: If you're unsure which measles vaccine — or how many doses — you received, it's better to be safe than sorry and roll up your sleeve.