Both killed and live attenuated measles vaccines became available in 1963. Live attenuated vaccine was used more often than killed vaccine. The killed vaccine was found to be not effective and people who received it should be revaccinated with live vaccine. Without a written record, it is not possible to know what type of vaccine an individual may have received. So, people born during or after 1957 who received killed measles vaccine or measles vaccine of unknown type, or who cannot document having been vaccinated or having laboratory-confirmed measles disease should receive at least 1 dose of MMR. Some people at increased risk of exposure to measles (such as healthcare professionals and international travelers) should receive 2 doses of MMR separated by at least 4 weeks.