evidence: Extremely rarely, a person may have a serious allergic reaction to MMR vaccine. Anyone who has ever had a life-threatening allergic reaction to the antibiotic neomycin, or any other component of MMR vaccine, should not get the vaccine. [...] - Measles inclusion body encephalitis, or severe brain swelling caused by the measles virus, is a complication of getting infected with the wild-type measles virus. While rare, this disorder almost always happens in patients with weakened immune systems. The illness usually develops within 1 year after initial measles infection and has a high death rate. There have been three published reports of this complication happening to people who are vaccinated. In these cases, encephalitis developed between 4 and 9 months after MMR vaccination. In one case, the measles vaccine strain was identified as the cause. [...] Because signs of autism may appear around the same time children receive the MMR vaccine, some parents may worry that the vaccine causes autism. Vaccine safety experts, including experts at CDC and the American Academy of Pediatrics (AAP), agree that MMR vaccine is not responsible for recent increases in the number of children with autism. In 2004, a report by the Institute of Medicine (IOM) concluded that there is no link between autism and MMR vaccine, and that there is no link between autism and vaccines that contain thimerosal as a preservative.