描述
**Measles vaccine** protects against becoming infected with
[measles](https://en.wikipedia.org/wiki/Measles). Nearly all of those
who do not develop immunity after a single dose develop it after a
second dose. When rates of
[vaccination](https://en.wikipedia.org/wiki/Vaccination) within a
[population](https://en.wikipedia.org/wiki/Population) are greater than
92%, outbreaks of measles typically no longer occur; however, they may
occur again if rates of vaccination decrease. The
[vaccine](https://en.wikipedia.org/wiki/Vaccine)\'s effectiveness lasts
many years. It is unclear if it becomes less effective over time. The
vaccine may also protect against measles if given within a couple of
days after exposure to measles.