Measles vaccine is a vaccine that prevents measles. After one dose at the age of nine months 85% are immune, while a dose at twelve months results in 95% immunity to measles. Nearly all of those who do not develop immunity after a single dose develop it after a second dose. When rates of vaccination within a population are greater than ~92% outbreaks of measles typically no longer occur; however, they may occur again if rates of vaccination decrease. The 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.
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