Measles vaccine is a vaccine that prevents 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.
The vaccine is generally safe, even for those with HIV infections. Most children do not experience any side effects. Side effects if they do occur are usually mild, such as fever, rash, pain at the site of injection, and joint stiffness; and are short lived. Anaphylaxis has been documented in about 3.5–10 cases per million doses.Rates of Guillain–Barré syndrome, autism and inflammatory bowel disease do not appear to be increased by measles vaccination.