adults born between 1963, when the first measles vaccine was approved, and 1968. During that period, some children received an inactivated (killed) measles vaccine that was less effective than the live vaccine.

People born before 1957 are considered to have “presumptive evidence” of immunity, because nearly everyone born during this period got the disease during childhood. But health-care workers born before 1957 who don’t have proof of immunity should consider getting the vaccine.

https://www.yalemedicine.org/news/should-you-get-a-measles-vaccine-booster