That's because when the measles vaccine first became available, in 1963, there were two versions and only one was effective. The killed vaccine was discontinued in 1967 when it was determined that it did not, in fact, protect against measles virus infection. In 1968, a new version of the live measles vaccine hit the market and is still in use today. Since 1971, the measles vaccine has been combined with the mumps and rubella vaccines in the three-in-one MMR vaccine. The CDC's Measles FAQ page says one dose of measles vaccine is about 93 percent effective at preventing measles if exposed to the virus.

Source: Forbes April 20, 2019 17:07 UTC