Why are adults born before 1966 immune to measles?

Adults born before 1966 are often considered immune to measles because they likely contracted the disease during childhood, prior to the widespread availability of the measles vaccine, which was introduced in 1963. Natural infection typically provides lifelong immunity. Additionally, many individuals in this age group experienced measles outbreaks before vaccination became common, further increasing their likelihood of immunity. As a result, they have developed antibodies against the virus through natural exposure.